Migrate codegen to shared build setup, remove package build pre-step from RNTester (#39540)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39540

This simplifies the use of Codegen when creating dev builds of `rn-tester` in the monorepo. It now runs from source for this internal scenario, and this package is now built using the shared monorepo build setup.

Changes:
- Migrate `packages/react-native-codegen` to the shared `yarn build` setup.
    - Update package to use `"exports"` field and wrap entry point modules with `babel-register` (NOTE: This is only required for each entry point internally used in the monorepo).
- Fixup small Flow syntax quirks that fail under `hermes-parser`.
- Remove `BuildCodegenCLITask` task from Android build.
- Remove Codegen `build.sh` call from iOS build, use `require.resolve` for `combine-js-to-schema-cli.js` entry point.

Externally significant FYIs:
- `react-native/codegen` is converted to use the `"exports"` field — it should export all `.js` files, as before.
- `codegenPath` is now ignored and marked as deprecated on `ReactExtensions.kt`.

NOTE: TypeScript auto-generation is not yet enabled on this package, since it uses CommonJS `module.exports` syntax (unsupported by `flow-api-translator`).

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D49370200

fbshipit-source-id: 992913155169912ea1a3cb24cb26efbd3f783058