Add visionOS sample to the project #6669

We would like to add a visionOS sample to the project, as a sibling to the tvOS sample (#5275).

This unblocks the visionOS build-only CI check tracked in #6648 — CI needs a sample app to compile RNSentry against, since it depends on React headers and can't be validated with a standalone pod lib lint.

Context

• visionOS is supported in React Native via @callstack/react-native-visionos.
• RNSentry.podspec already declares s.visionos.deployment_target, so the native pod targets the platform — this sample proves it end-to-end.

Tasks

• Scaffold a visionOS sample app (react-native-visionos)
• Wire up @sentry/react-native init
• Verify it builds against the local SDK
• Feed it into the visionOS CI gate (Add CI checks for tvOS and visionOS #6648)