Add React Native Web support

[dewana-sl]

Summary

Adds React Native Web support while keeping the existing native iOS and Android paths intact.

Changes

• Adds browser export and browser field mappings for web bundlers.
• Adds web-specific platform files for SDK setup, model download/cache, ONNX Runtime Web inference, CE phonemizer asset loading, browser playback, NPZ loading, and bundled assets.
• Adds Cache API, memory, and Node filesystem asset storage helpers for the web path.
• Adds onnxruntime-web as a runtime dependency.
• Exposes createBrowserAudioPlayer() plus web-compatible Expo and React Native Sound helper shims for shared app code.
• Loads ONNX Runtime safely in browser builds so Expo web consumers do not need example-specific Metro config.
• Updates README, getting started docs, playback docs, changelog, and agent guidance for web support.

Validation

• npm run typecheck
• npm test
• npm run build
• npm pack --pack-destination /tmp
• Expo word timing example web bundle smoke test
• Browser runtime check with the unchanged Expo word timing example