Add support for building using Metro (#44464)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44464

Adds `app` to allow building and serving your React Native app in a similar structure to the boostrap and build tasks.  This is the more comprehensive followup to D57067040.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D57067039

fbshipit-source-id: fdbe891657d826535cb779a4d1b71cfd13921684