Follow-up to https://github.com/vercel/next.js/pull/17538 per https://github.com/vercel/next.js/pull/17538#discussion_r499647323 this updates the check to use the existing `hasNextSupport` export instead of checking the environment variable directly

This makes sure to normalize the params for optional catch-all routes on Vercel since for `fallback: true` pages the `[[...paramName]]` value will be provided for the undefined/root param which needs to be normalized. 

Tests have been added in https://github.com/vercel/vercel/pull/5247 and were manually tested with the changes in this PR with https://github.com/ijjk/next-update-loader

Fixes: https://github.com/vercel/next.js/issues/17220
x-ref: https://github.com/vercel/vercel/pull/5247

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. 

Noticed this method was left-over from previous query handling logic in `api-utils` while working on https://github.com/vercel/next.js/pull/17370 so this removes the extra code which should be safe since `api-utils` is an internal file that shouldn't be relied on externally. 

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

* Expose dotenv loading under separate package

* Update pre-compiled

* Rename package to @next/env

* Update lint ignores

* Update package.json
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

Noticed while adding config checks for a new config that the `basePath` checks were wrapped in a `result.experimental` check and even though this should always be true from the default experimental value being an object the `basePath` checks shouldn't be wrapped in this check since it isn't experimental anymore. 

There was a blank line, this PR removes it :D

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` 😅 .

currently, the `no-html-link-for-pages` eslint rule will look for the pages folder in either `pages`, or a custom folder provided via rule option. this PR adds support for also looking in `src/pages` by default.

(sidenote: not sure about the custom pagesfolder path: does next support locations other than `pages` and `src/pages`)?)

fixes  #16426

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

On the latest beta of webpack 5 resolving fails with the below error and according to https://github.com/webpack/webpack/issues/11467 is due to the imports in this module not being fully specified. This adds the config mentioned in the thread to correct the resolving for this module. 

```sh
Failed to compile.
--
16:33:50.046 | ModuleNotFoundError: Module not found: Error: Can't resolve './assertThisInitialized' in '/vercel/f03cc85/node_modules/@babel/runtime/helpers/esm'
16:33:50.046 | > Build error occurred
16:33:50.047 | Error: > Build failed because of webpack errors
16:33:50.047 | at build (/vercel/f03cc85/node_modules/next/dist/build/index.js:15:918)
16:33:50.099 | error Command failed with exit code 1.
```
 

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

Earlier today #17038 was merged which I opened to fix a problem when using `webpack@5.0.0-beta.30` with Next.js using the new Webpack 5 support. In that PR, the only change was the renaming of a configuration key. I later discovered that the change on the Webpack side was different than I initially thought, and this meant that the fix I submittted to Next.js didn't work.

This PR intends to fix the remaining problems. Webpack 5 now accepts a `environment` key that can be used to configure the target output. Previously, this was known as `ecmaVersion` and accepted a number. Now, `environment` accepts a configuration object with individual options. I've configured this in such a way where it resembles an ES5 environment:
```js
environment: {
   arrowFunction: false,
   bigIntLiteral: false,
   const: false,
   destructuring: false,
   dynamicImport: false,
   forOf: false,
   module: false,
}
```

For some reason babel-plugin-syntax-jsx of babel 6 was used instead of
babel 7 version.

This PR fixes #17035. As described in the issue, there was a breaking change in `webpack@5.0.0-beta.30`: `output.ecmaVersion` was replaced by `output.environment`. This meant Next.js apps using this `webpack` version would break. This PR updates the relevant Webpack config.

I think this will break any apps that are still using `webpack@5.0.0-beta.29`, but I don't know whether that is a problem as this is a beta feature. If it is, I'd love it if someone could let me know how to detect beta versions in the code so I can make it backwards-compatible.

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

ref https://github.com/GoogleChrome/web-vitals/pull/68

won't fail the new [`no-unload-listeners`](https://github.com/GoogleChrome/lighthouse/pull/11085) Lighthouse audit.

Ref https://github.com/webpack-contrib/terser-webpack-plugin/blob/v4.1.0/package.json#L44

cacache is a dependency of terser-webpack-plugin. The latest version
depends on cacache 15 while next adds cacache 13. This may give
unexpected results. Better keep it in sync with terser plugin.

Babel-preset-env includes includes optional chaining and
nullish-coalescing since [7.8.0](https://github.com/babel/babel/releases/tag/v7.8.0).

In this diff I removed these plugins from next preset to prevent
dependency duplication when their newer versions are out.

The new version replaced big clone-deep package with dependency-free klona
- https://github.com/webpack-contrib/sass-loader/releases/tag/v10.0.0
- https://github.com/webpack-contrib/sass-loader/releases/tag/v9.0.0

Also deduped some related transitives.