fix: resolve native SafeAreaProvider via exports map (#279)

The ./components/* wildcard in the exports map resolved
env(safe-area-inset-*) to the web passthrough file (.tsx) instead
of the native wrapper (.native.tsx) that injects CSS variables via
SafeAreaEnv. Metro treats exports-resolved paths as final and does
not apply platform-suffix resolution.

Add an explicit exports entry for
./components/react-native-safe-area-context that maps the
react-native condition to .native.tsx, and add a SafeAreaProvider
convenience re-export consistent with the component naming pattern
used by View, Text, etc.

Enable the previously skipped env(safe-area-inset-*) test.