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
