Planes detection API

Chrome is starting with an experimental [Planes detection API](https://github.com/immersive-web/real-world-geometry/blob/master/plane-detection-explainer.md). Example [here](https://storage.googleapis.com/chromium-webxr-test/r672724/proposals/phone-ar-plane-detection.html).

We should update Exokit's implementation to match. 🦄 