fix: move locale detection to netlify redirects

[ascorbic]

Summary

Currently, locale detection requires the request to reach the handler function, which means it breaks for static or ODB routes. This PR moves the detection into Netlify redirects, so it happens before other redirects are processed.

This PR also refactors the redirects into a different file, because it was getting unwieldy keeping it inside the config helper

Test plan

1. TODO: create a test site with a static or fallback page at the root
2. Visit the page with a browser locale of "es"
3. Observe a redirect to /es/

Relevant links (GitHub issues, Notion docs, etc.) or a picture of cute animal

Fixes #859

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• Add docs when necessary

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[tiffafoo]

So many ignores

[ascorbic]

I'm going to fix the @netlify/build types

[ascorbic]

netlify/build#3908

[netlify]

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[ascorbic]

I'm going to merge this, because even though the libredirect issue means the autodetection based on browser header isn;t working, it's better than the current behaviour

[andykenward]

@ascorbic Is the next.js built in autodetection still not supported?