Add debugger-frontend package and manual build script (#39161)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39161

Changelog: [Internal]

Creates a new stub package named `react-native/debugger-frontend`, which will contain a minimal build of the Chrome DevTools frontend adapted for React Native. Initially, the compiled frontend assets will be checked into the React Native repo, but we intend to replace this with a fully automated build in CI at a later date.

Reviewed By: huntie

Differential Revision: D48680624

fbshipit-source-id: a24c8b019881187963d0be88e773bc0a97a2437d