This ensures redirects are handled properly with i18n + `trailingSlash: true`, additional tests have also been added to ensure this is covered 

Fixes: https://github.com/vercel/next.js/issues/19069

This mirrors the `basePath: false` behavior for custom-routes with `locale: false` to allow users to configure the locales manually (`locale: false`) or have the locales be handled automatically for custom-routes. 

Fixes: https://github.com/vercel/next.js/issues/18927
Fixes: https://github.com/vercel/next.js/issues/18795

This ensures we don't detect the locale from the header when `localeDetection` is set to `false`

Fixes: https://github.com/vercel/next.js/issues/18482

This makes sure custom-routes function properly with i18n support. Since the `locale` is stripped from the `pathname` this re-adds when checking custom-routes to allow the locale to be matched against

Closes: https://github.com/vercel/next.js/issues/18349

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

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 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