Relocate babel-register script (#49102)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49102

Moves this script one level up. In the next diff, will be used to support execution of scripts themselves, as well as `packages/`.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D68960279

fbshipit-source-id: 7b62420c269dc1c1366ac9a827db078d34cb86c5