Add type exports to index.js.flow (#50225)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50225

Updates `index.js.flow` to export (what should be) all public types for `react-native`, closely matching the resolved exported types of `types/index.d.ts` (current manual TS defs).

- Note that this first pass doesn't have to be exhaustive (not yet load bearing) — however, I've done a best-effort scan, which passes our current `__typetests__` fixtures.

**Approach: Explicit type imports in `index.js.flow`**

We have a number of options for how we organise and re-export values and types from the new index file — for now, we're opting for explicitly exporting each symbol here.

- While this clutters the index file somewhat, it also provides a single scannable source of truth — without introducing / altering each contributing module's type exports (we might want to do this eventually, but we're far from having strong organisation denoting the public API boundary). It's self-documenting that the index file contains all root exports by name.
- At the same time, we currently have some exceptions that use `export [type] *` to line up with TypeScript `namespace`s. We aim to review and update these within this release cycle, to enforce the above system.

Changelog: [Internal] - The `react-native` package now exports all public types in Flow (fbsource)

Reviewed By: rubennorte

Differential Revision: D71741575

fbshipit-source-id: 770e2b490e494ee195f4240358fec39f69145a94