feat(metro): use custom babel-transformer (#23607)

# Why

Historically, we've avoided using a custom Metro babel transformer
because some users leverage this value to transform certain files.
Ho

feat(metro): use custom babel-transformer (#23607)

# Why

Historically, we've avoided using a custom Metro babel transformer
because some users leverage this value to transform certain files.
However, we need to be able to pass custom options to the babel
configuration like `metroTarget` and we also need to ensure
`babel-preset-expo` is always used. These two aspects ensure Node.js
environments can be accounted for in `babel-preset-expo`.

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# How

- Fork the upstream babel-transformer.
- Modify the fork to use babel-preset-expo by default when a
`babel.config.js` isn't defined.
- Expose the `metroTarget` field to the Babel caller. This is either
`client`, `node`, or `undefined`. When defined, we'll use this to strip
server code, and use more focused babel transforms (not yet
implemented).
- Expose the fork under `@expo/metro-config/babel-transformer`.
- Add docs on how to extend the Expo metro babel transformer.
- Delete the previous wrapper that was used when no babel.config.js file
was defined.

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# Test Plan

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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 `npx expo prebuild` & EAS Build
(eg: updated a module plugin).

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

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