Add unstable-internals entry point

[huntie]

Summary:
See RFC0894: Removing deep imports from react-native

Adds a 'react-native/unstable-internals-do-not-use' entry point that acts as an explicit, opt-in escape hatch for frameworks and libraries that depend on specific React Native internals.

Follows (and supersedes) react-native-community/discussions-and-proposals#985.

Resolves T270727304.

Motivation

• Maintains a known list of which React Native internals are still in use.
• Decouples removal of the Strict API opt-out from the longer-term decisions about these APIs.
• Lets us relocate supporting source files freely, since consumers reference this subpath instead of deep internal paths.

Notes

• Reuses the runtime object-with-getter pattern (same as index.js), to avoid any native module side effects caused by loading adjacent file exports.
  • e.g. in Expo, hmrUtils.js is designed to be run early in RN startup — preserve the lazy require characteristic and do not break this.
• TypeScript defs on this API are minimal, with complex internal input/handle types reduced to unknown. As with D110602194, these are intended to be feature-locked.

Exposed internals and known consumers

NOTE: The exports the community needs have reduced slightly since the spreadsheet in RFC0895 and would almost all align with devsupport. However, we're explicitly leaving a decision to formalise until later, and we may need to add in extra APIs quickly based on RC feedback.

Export	Source module	Known consumers
AppContainer	Libraries/ReactNative/AppContainer	react-native-screens (DebugContainer.tsx)
AssetSourceResolver	Libraries/Image/AssetSourceResolver	Re.pack (IncludeModules.ts)
customDirectEventTypes	Libraries/Renderer/shims/ReactNativeViewConfigRegistry	react-native-gesture-handler (customDirectEventTypes.ts)
DevLoadingView	Libraries/Utilities/DevLoadingView	Expo (hmrUtils.native.ts), Re.pack (WebpackHMRClient.ts)
getDevServer	Libraries/Core/Devtools/getDevServer	Expo (getDevServer.native.ts, hmrUtils.native.ts, base.ts, getDevServer.native.ts, devServerEndpoints.ts, getConnectionInfo.native.ts, index.native.ts)
HMRClient	Libraries/Utilities/HMRClient	Expo (metroServerLogs.native.ts)
NativeExceptionsManager	Libraries/Core/NativeExceptionsManager	Expo (hmrUtils.native.ts), Re.pack (WebpackHMRClient.ts)
NativeRedBox	Libraries/NativeModules/specs/NativeRedBox	Expo (hmrUtils.native.ts), Re.pack (WebpackHMRClient.ts)
NativeSourceCode	Libraries/NativeModules/specs/NativeSourceCode	Expo (getBundleUrl.native.ts)
PressabilityDebugView	Libraries/Pressability/PressabilityDebug	react-native-gesture-handler (PressabilityDebugView.tsx)

Alternative names

• unstable-frameworks
• unstable-devsupport

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D110911864

[meta-codesync]

@huntie has exported this pull request. If you are a Meta employee, you can view the originating Diff in D110911864.