feat: i18n locale redirects support

[james-elicx]

This PR does the following:

• Fixes an issue where redirects weren't breaking out of routing at the right point.
• Adds a utility function for parsing the accept-language header value.
• Introduces support for the source route locale option and the next.config.js option for i18n.
• Adds tests for the new functionality.

It is somewhat of a continuation of #222 as that fixed a major blocker for i18n.

So, there is a little quirk with how the locale redirects have to be handled. There are two different types of ways internationalization works. These are sub-path routing and domain routing. The build output config route.src value for sub-path routing is simply /, which causes issues.

The Next.js docs state that automatic locale detection occurs when a user visits the root of the website, which would mean that the regex should be ^/$, except it isn't. Therefore, we have to add custom behaviour to check whether the route.src is a regex, and if it isn't, check that it is equal to the current path before determining whether we should apply the locale redirects.

Sub-path routing entry:

{
	src: '/',
	locale: {
		redirect: { en: '/', fr: '/fr', nl: '/nl', es: '/es' },
		cookie: 'NEXT_LOCALE',
	},
	continue: true,
}

Domain routing entry:

{
	src: '^//?(?:en|fr|nl|es)?/?$',
	locale: {
		redirect: { es: 'https://example.es/' },
		cookie: 'NEXT_LOCALE',
	},
	continue: true,
}

As you can see, the two values for route.src are very different. I find this to be rather bizarre behaviour, and to me, it looks like the Vercel build output isn't generating the correct matching patterns. Anyway, so to address the quirks, I have done what I detailed above.

Also tested it using test apps for plain Next.js i18n, next-translate, and next-i18n. The test app shouldn't actually matter anyway, because they all use next.config.js i18n under the hood, which is what this PR introduces better support for.

closes #28
closes #37
closes #157

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🦋 Changeset detected

Latest commit: 956c38e

The changes in this PR will be included in the next version bump.

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@cloudflare/next-on-pages	Minor

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[github-actions]

🧪 A prerelease is available for testing 🧪

You can install this latest build in your project with:

npm install --save-dev https://prerelease-registry.devprod.cloudflare.dev/next-on-pages/runs/4920020835/npm-package-next-on-pages-227

Or you can immediately run this with npx:

npx https://prerelease-registry.devprod.cloudflare.dev/next-on-pages/runs/4920020835/npm-package-next-on-pages-227

[james-elicx]

We identified an issue where there is a record in the miss phase that rewrites all requests containing the default locale to /.

To prevent this behaviour that we don't want, we modify the source route's src regex to be locale friendly using any of the previously found locales.

Problematic record example:

{
  "src": "/en",
  "dest": "/",
  "check": true
},

This should instead have an src value of ^/en$, so that it only remaps paths for exactly the default locales to /.

[dario-piotrowicz]

So awesome!!! thank you very much! ❤️