Re-expose src/ paths via exports (default) (#51699)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51699

Re-expose **all `"./src/*"` paths via `package.json#exports`.

Follows D72228547 — as we'd attempted to be proactive with hiding `src/`, but are now reverting — essentially reducing this change as much as possible (read: most defensive/safer effect on OSS).

**Motivation**

Mitigates a warning emitted by Metro that may surface in new bare React Native template projects.

{F1978757796}

This is due to a 1P reference to `'react-native/src/private/featureflags/ReactNativeFeatureFlags` in `react-native/virtualized-lists`.

This is enough motivation to undo part of our change with introducing `"exports"` on `react-native` in 0.80.

**Especially**, to avoid confusion with other warnings we've intentionally introduced in 0.80 around subpath imports.

- The key difference is the above screenshot is from Metro and covers **any** import from any `node_modules` file — as opposed to just the immediate project.

Changelog:
[General][Changed] - Re-expose `src/*` subpaths when not using the Strict TypeScript API

The net breaking change for 0.80, once picked, is simply the presence of `"exports"` (only very edge case effects on expanding per-platform extensions, which we've mitigated). **All exported paths equivalent**.

Reviewed By: robhogan

Differential Revision: D75682566

fbshipit-source-id: f90c7298279c6be3a4eab70f2dfdc618ffcf1124