feat(metro-config, cli): CSS support for Metro web (#21941)

# Why

This PR moves the CSS support from Expo Router over to
`expo/metro-config` behind a beta flag. This is because we need deeper
i

feat(metro-config, cli): CSS support for Metro web (#21941)

# Why

This PR moves the CSS support from Expo Router over to
`expo/metro-config` behind a beta flag. This is because we need deeper
integration with Expo CLI and Metro in order to emit static CSS files in
production bundles. CSS is required for media queries -> rehydration.

- Related: https://github.com/expo/router/pull/397
https://github.com/expo/router/pull/223

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# How

Instead of a babel transformer, we now use a custom "transformer" which
gives us the ability to add extra metadata to a Metro module on export.
This means we can pass the raw and processed CSS for writing to disk.

The development version of CSS still uses script injection via
JavaScript, meaning static rendering cannot currently be tested in
development.

CSS Modules are implemented (web-only currently) using lightningcss. The
export is generated to work with React Native for web. Consider the
following block:

```js
export default { ...StyleSheet.create({ container: { $$css: true, _: 'hashed-container-id' } }) }
```

CSS Variables are not currently hashed, enabling the user to define
variables in a global CSS file and access them in CSS Modules (subject
to change in order to support native). CSS variables are accessible from
the default export as strings: `styles['--color'] === '--color'`.


## Docs

I've chosen to document in the versioned metro doc instead of the
unversioned guide, this seems like a reasonable spot given the highly
experimental nature of this feature.


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# Test Plan

- Unit tests for conversion.
- [x] Test in/out for transformer.
- [ ] Test static export.
- TBD for 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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