Move metro-config package into monorepo build, enable TS generation (#41836)

Summary:
This adds `react-native/metro-config` to the monorepo build tool and emits the missing typescript declarations.

Right now, we do have typescript declarations on `metro-config`, but not `react-native/metro-config`. Which makes everything a bit harder extend from "[the default React Native metro config](https://github.com/facebook/react-native/pull/36502)" in Expo.

> Note, I also added the same `exports` block from `react-native/dev-middleware` for conformity.

One open question here is, why aren't we exporting _all_ helper functions from `metro-config`? To me, its a bit weird that we need both `metro-config` _and_ `react-native/metro-config` as `loadConfig` isn't exported.

## Changelog:

[INTERNAL] [FIXED] - Emit typescript declaration files for `react-native/metro-config`

Pull Request resolved: https://github.com/facebook/react-native/pull/41836

Test Plan:
Run the build tool, and check if the typescript declarations are emitted for `react-native/metro-config`.

```
yarn build metro-config
```

Reviewed By: hoxyq

Differential Revision: D51943453

Pulled By: huntie

fbshipit-source-id: cfaffe5660053fc9a9fcbe3dacf7f6ccc2bde01b