feat(metro): use custom babel-transformer

[EvanBacon]

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.

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.

Test Plan

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[amandeepmittal]

Lgtm from docs perspective 👍

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