dillon1000/react

[DevTools] Updated source map extension format + more precise types (#22073)

## Summary

Follow up from https://github.com/facebook/react/pull/22010.

As suggested by @motiz88, update the way the react sources metadata is stored within the fb sources metadata. Specifically,  instead of `x_facebook_sources` directly containing a hook map in the second position of the metadata tuple for a given source, it contains the react sources metadata itself, which is also a tuple of react sources metadata for a given source, and which contains the hook map in the first position. This way the react sources metadata tuple can be extended to contain more react-specific metadata without taking up more positions in the top-level facebook sources metadata.

As part of this change:
- Adds more precise Flow types, mostly borrowed from Metro
- Fixes the facebook sources field name (we were using `x_fb_sources` but it should be `x_facebook_sources`

## Test Plan

- yarn flow
- yarn test
- yarn test-build-devtools