feat(metro-config): add postcss (#22032)

# Why

- Required for modifying CSS and supporting Tailwind on web (but also
universally).
- Eventually we'll want to add some postcss transforms by defa

feat(metro-config): add postcss (#22032)

# Why

- Required for modifying CSS and supporting Tailwind on web (but also
universally).
- Eventually we'll want to add some postcss transforms by default
(standard in the web community).

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# How

- Write a `postcss` parser which loads the user config.
- `postcss.config.js` and `postcss.config.json` are the only supported
config files.
https://github.com/webpack-contrib/postcss-loader/blob/ef332db57381ed1d8aaa4a9a20eec0e0b21fdb7c/src/utils.js#L64-L114
- Process input CSS with postcss config and pass output to standard CSS
loading process.
- TBD: More advanced Metro cache invalidation.
- We currently have a basic hashing system which auto invalidates the
transformer when the config file and browserslist options change.


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# Test Plan

- Added minor tests for certain parts of the flow but nothing e2e.

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# Checklist

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- [ ] Documentation is up to date to reflect these changes (eg:
https://docs.expo.dev and README.md).
- [ ] Conforms with the [Documentation Writing Style
Guide](https://github.com/expo/expo/blob/main/guides/Expo%20Documentation%20Writing%20Style%20Guide.md)
- [ ] This diff will work correctly for `expo prebuild` & EAS Build (eg:
updated a module plugin).

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Co-authored-by: Expo Bot <[email protected]>

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