Enable Strict TS API by default (#57490)

Summary:
Pull Request resolved: https://github.com/react/react-native/pull/57490

See [**RFC0894: Removing deep imports from react-native**](https://github.com/react-native-community/discussions-and-proposals/pull/894)

This is the **big switch** to enable the Strict TypeScript API (generated types + single index entry point) by default in React Native.

**Opt-in → opt-out**

After this change, the main `react-native` package resolves its `"types"` entry points only to `types_generated/index.d.ts` — with no other subpaths available.

The new `"react-native-legacy-deep-imports"` condition maps to legacy `types/` and `Libraries/*.d.ts` sources.

**Impact limitation**: For this stage of rollout, the `"default"` condition continues to resolve to source files. Only TypeScript is affected.

**How to opt out**

Opposite of today's opt-in, which we will update in [the docs](https://reactnative.dev/docs/strict-typescript-api). Again, the only impact area today is **TypeScript**.

```json5
// tsconfig.json
{
  "extends": "react-native/typescript-config",
  "compilerOptions": {
    ...
    "customConditions": ["react-native-legacy-deep-imports"]
  }
}
```

**Other changes**

- Drop `react-native/typescript-config/strict` entry point, update README.
- Update `__typetests__`.

**Rollout plan**

**Target release: 0.87**. This and the contributing stack will be cherry picked for RC1.

- We've conducted testing against 100+ real Expo codebases, giving us the confidence that we've reduced breaking changes enough that the vast majority of RN codebases can migrate.
- The Strict API includes a number of **intentional breaking changes**, and docs have been kept up to date.
- We're shipping a `/migrate-to-strict-api` skill to migrate via agents, see https://github.com/react-native-community/skills/pull/3.

**What's improved since 0.80?**

Since the initial opt-in launch of the Strict API in 0.80, we've been making continuous improvements over the last year to get our generated types into a widely launchable state.

Most notably:

- 21+ new/updated root APIs and fixes due to community feedback ([discussion](https://github.com/react-native-community/discussions-and-proposals/discussions/893), [PRs](https://github.com/react/react-native/pulls?q=is%3Apr%20label%3A%22JS%20API%20stabilization%20(1.0)%22%20is%3Aclosed)).
- Upstream encapsulation blocker in TypeScript, fixed in 6.0 (https://github.com/react/react-native/issues/53565).
- Tailwind/Uniwind compatibility (`interface` types for props).
- `*Instance` ref type exports for all built-in components (https://github.com/react-native-community/discussions-and-proposals/pull/1003).
- Fixes to previously mistyped, high impact APIs, such as `Appearance`.
- New subpath entry points for `asset-registry`, `setup-env`, and others.
- Refinements to doc comments/type translation build.

**Rollback plan**

Revert this diff.

IMPORTANT: We'll adopt a policy of **super-eager rollback**, if there are any unsolvable issues during the RC phase.

Changelog:
[General][Breaking] - React Native's default JavaScript API is now the [Strict TypeScript API](https://reactnative.dev/docs/strict-typescript-api). Use `customConditions: ["react-native-legacy-deep-imports"]` to opt out.

Reviewed By: cortinico

Differential Revision: D110458670

fbshipit-source-id: 4b0e0b458a5f895f783d6d936e7b11ccff2df076

scripts/run-ci-javascript-tests.js

@@ -81,7 +81,7 @@ try {
   );
 
   describe('Test: TypeScript tests');
-  execAndLog(`${YARN_BINARY} run test-typescript`);
+  execAndLog(`${YARN_BINARY} run test-typescript-legacy`);
 } catch (e) {
   if (e instanceof ExecError) {
     console.error(e.message);