Update common interface shadowing for type generation (#49205)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49205

Changelog: [Internal]

Updates how name shadowing works for the TS type generation prototype to align more with how Flow does it - `.js.flow` files shadow every other file with the same name, then `.js` file (if exists) is treated as the common interface.

The script still uses `.flow.js` for common interface, which will be changed in another diff.

Reviewed By: huntie

Differential Revision: D68958772

fbshipit-source-id: caa390711f2bcd7666d875703fc316d874500a0d