feat: JS-only implementation on top of core safe area primitives

[janicduplessis]

Summary

Prototype of what the library looks like on top of the core safe area primitives proposed in react/react-native#57967 — opened for discussion, not for landing: it gates on a placeholder version (RN ≥ 0.88) and can only run against a React Native checkout that includes that PR.

With core reporting insets itself, the library needs no native code:

• SafeAreaProvider renders a plain View with the core onSafeAreaInsetsChange prop. The payload is identical to our onInsetsChange, so nothing else changes. Also covers SafeAreaListener.
• SafeAreaView promotes the existing web implementation to every platform: insets come from the provider context and are applied per-edge as padding or margin in JS, with the same edges/mode semantics as before.
• initialWindowMetrics reads Dimensions.get('window').safeAreaInsets, which the core PR exposes using the same native inset computation as the view prop, so the RNCSafeAreaContext module is not needed either.

The part that makes the JS implementations viable is that core dispatches the inset event synchronously: the padding derived from the insets is mounted in the frame the insets changed in, so the JS SafeAreaView keeps the no-flicker behavior that previously required the native shadow node.

On React Native versions before the core prop exists, everything falls back to the current native path — SUPPORTS_CORE_SAFE_AREA_INSETS in src/coreSafeAreaInsets.ts is the single gate.

Test Plan

• yarn validate:jest — 22 tests pass. The jest environment reports RN 1000.0.0, so the suite exercises the new JS path (snapshots updated accordingly); the legacy native path is covered by mocking the gate.
• yarn validate:typescript passes.
• Not run against a real app: that requires a React Native build containing Add an onSafeAreaInsetsChange view prop react/react-native#57967. End-to-end behavior of the underlying primitive — including frame-by-frame validation of no-flicker sync rendering on both platforms — is in that PR.