Move experimental type generation into dedicated script (#48867)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48867

Also:
- Rename output dir to `types_generated/`.
- Support source patterns outside of the `react-native` package.

Changelog: [Internal]

Reviewed By: j-piasecki

Differential Revision: D68498211

fbshipit-source-id: 7f9f540efd6d3d2c70b8b1721738f3fea569641d