Unify the confusing `config.images.sizes` ([Image Optimizer](https://github.com/vercel/next.js/discussions/17141)) and `config.images.breakpoints` ([Image Component](https://github.com/vercel/next.js/discussions/16832)).

Co-authored-by: NJJ Kasper <jj@jjsweb.site>

This makes sure the correct `initialRevalidateSeconds` field is populated in the `prerender-manifest` for non-dynamic SSG pages since they will be inserted into the `initialPageRevalidationMap` under their locale prefixed variant with `i18n` enabled

x-ref: https://github.com/vercel/next.js/pull/17370

This PR adjust the Web Vitals reporting to be called *after* rendering occurs. It used to work like this, but React's render method is no longer synchronous—so we have to do it in an effect.

Existing tests should cover this code path, and IMO it's unfeasible to test that it's invoked _after_ hydration.

Also removed the `&& ST` condition which is not relevant to Web Vitals.

* Ensure i18n support with AMP

* Fix type error

* Update size-limit

This adds the i18n config items to the routes-manifest so that they can accessed in the builder. This doesn't increment the routes-manifest version as existing value shapes haven't been modified and an additional non-breaking value is being added

x-ref: https://github.com/vercel/next.js/pull/17370

Co-authored-by: NTim Neutkens <tim@timneutkens.nl>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

This adds the `locale` prop for `next/link` to allow transitioning between locales client-side and also allows passing the locale to `router.push/replace` via the transition options similar to `shallow` e.g. `router.push('/another', '/another, { locale: 'nl' })`

x-ref: https://github.com/vercel/next.js/pull/17370

This updates to set the `NEXT_LOCALE` cookie to the default locale when the user prefers a different locale from the default in their `accept-language` header but visits the default locale path e.g. `/en-US` with a `accept-language` preferred header of `nl` will set the `NEXT_LOCALE=en-US` header and then redirect to `/`

x-ref: https://github.com/vercel/next.js/pull/17370

Co-authored-by: NLuis Alvarez D <luis@vercel.com>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

Co-authored-by: NTim Neutkens <timneutkens@me.com>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Co-authored-by: NJJ Kasper <jj@jjsweb.site>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

While working on https://github.com/vercel/next.js/pull/17755 noticed a couple of cases that needed fixing and broke them out to this PR to make that one easier to review. One fix is for `ssgCacheKey` where it wasn't having the `locale` prefix stripped correctly due to the locales no longer being populated under the server instances `renderOpts` and the second fix is for the `asPath` not being set to `/` when the `locale` is the only part in the URL e.g. `/en` became an empty string `""`

x-ref: https://github.com/vercel/next.js/pull/17370

Follow-up to https://github.com/vercel/next.js/pull/17370 this makes sure the locale detection is case-insensitive. 

Follow-up to https://github.com/vercel/next.js/pull/17370 this adds mapping of locales to domains and handles default locales for specific domains also allowing specifying which locales can be visited for each domain. 

This PR also updates to output all statically generated pages under the locale prefix to make it easier to locate/lookup and to not redirect to the default locale prefixed path when no `accept-language` header is provided. 

Follow-up PR to #17370 this adds generating auto-export, non-dynamic SSG, and fallback pages with all locales. Dynamic SSG pages still control which locales the pages are generated with using `getStaticPaths`. To further control which locales non-dynamic SSG pages will be prerendered with a follow-up PR adding handling for 404 behavior from `getStaticProps` will be needed. 

x-ref: https://github.com/vercel/next.js/issues/17110

This is a follow-up to https://github.com/vercel/next.js/pull/16973 which adds handling for the breaking change in the latest version of css-loader that causes unresolved file references in `url` or `import` to cause the build to fail. This fixes it by adding our own resolve checking and when it fails disabling the `css-loader`'s handling of it. 

Fixes: https://github.com/vercel/next.js/issues/17701

Follow-up PR to https://github.com/vercel/next.js/pull/17370 when the path is not prefixed with a locale and the default locale is the detected locale it doesn't redirect to locale prefixed variant. If the default locale path is visited and the default locale is visited this also redirects to the root removing the un-necessary locale in the URL. 

This also exposes the `defaultLocale` on the router since the RFC mentions `Setting a defaultLocale is required in every i18n library so it'd be useful for Next.js to provide it to the application.` although doesn't explicitly spec where we want to expose it. If we want to expose it differently this can be updated. 

This adds the initial changes outlined in the [i18n routing RFC](https://github.com/vercel/next.js/discussions/17078). This currently treats the locale prefix on routes similar to how the basePath is treated in that the config doesn't require any changes to your pages directory and is automatically stripped/added based on the detected locale that should be used. 

Currently redirecting occurs on the `/` route if a locale is detected regardless of if an optional catch-all route would match the `/` route or not we may want to investigate whether we want to disable this redirection automatically if an `/index.js` file isn't present at root of the pages directory. 

TODO: 

- [x] ensure locale detection/populating works in serverless mode correctly
- [x] add tests for locale handling in different modes, fallback/getStaticProps/getServerSideProps

To be continued in fall-up PRs

- [ ] add tests for revalidate, auto-export, basePath + i18n
- [ ] add mapping of domains with locales
- [ ] investigate detecting locale against non-index routes and populating the locale in a cookie

x-ref: https://github.com/vercel/next.js/issues/17110

Added a test case to make sure that creating a new project within the current directory works as expected.

This adds a warning when `next export` and custom routes are defined  outside of a platform that supports them since they won't be applied anymore. 

This handles the case where the children on a head element are undefined and not a string or an array of strings. This doesn't currently handle sub-children on head elements so additional handling will be needed if this is a feature we would like to support although can be discussed/investigated separately from this fix. 

Fixes: https://github.com/vercel/next.js/issues/17364  
Fixes: https://github.com/vercel/next.js/issues/6388
Closes: https://github.com/vercel/next.js/pull/16751

Prior to this pull request, Next.js would immediately decode all URLs sent to its server (via `path-match`).

This was rarely needed, and Next.js would typically re-encode the incoming request right away (see all the `encodeURIComponent`s removed in PR diff). This adds unnecessary performance overhead.

Long term, this will also help prevent weird encoding edge-cases like #10004, #10022, #11371, et al.

---

No new tests are necessary for this change because we've extensively tested these edge cases with existing tests.
One test was updated to reflect that we skip decoding in a 404 scenario.

Let's see if all the existing tests pass!

This PR fixes `info  - undefined (N/N)` log for `export` cli.

![image](https://user-images.githubusercontent.com/6259812/93186247-5d801500-f779-11ea-89ec-e20939d7b7c1.png)

Default parameter `label` for `createProgress()` was always ignored by `${Log.prefixes.info} undefined` 😅 .

This makes sure we have the correct `asPath` value to prevent breaking hydration for `getServerSideProps` pages and doesn't re-use the `resolvedUrl` value for the `asPath` and instead creates a separate `resolvedAsPath` value that only removes the `_next/data` prefix from the path. Additional tests have been added in the `getServerSideProps` suite to ensure correct `asPath` with rewrites. 

Fixes: https://github.com/vercel/next.js/issues/17113

Fixes SSG pages that start with `/api` not being detected as SSG pages. This also adds tests to ensure this is working correctly in the `prerender` suite.

x-ref: https://github.com/vercel/next.js/issues/17091 

This continues off of https://github.com/vercel/next.js/pull/17081 and provides this normalized `asPath` value in the context provided to `getServerSideProps` to provide the consistent value since the request URL can vary between direct visit and client transition and the alternative requires building the URL each time manually. 

Kept this change separate from https://github.com/vercel/next.js/pull/17081 since this is addressing a separate issue and allows discussion separately. 

Closes: https://github.com/vercel/next.js/issues/16407

This normalizes the `asPath` for `getServerSideProps` and `getStaticProps` pages to ensure it matches the value that would show on the client instead of a) the output pathname when revalidating or generating a fallback or b) the `_next/data` URL on client transition. 

Fixes: https://github.com/vercel/next.js/issues/16542

This uses the "Expand directories" feature introduces in Prettier 2.0 to automatically format all supported file types.

Also, I fixed some badly formatted files.

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

Closes: https://github.com/vercel/next.js/issues/14433

This adds an error when interpolation fails to make sure invalid `href`s aren't accidentally used and an invalid URL is built. 

Closes: https://github.com/vercel/next.js/issues/16944

Fixes https://github.com/vercel/next.js/issues/16147