Hit test spec draft - feedback appreciated

Hi all,

I have merged the PR that brings in hit test specification draft. Latest
version can be found at https://immersive-web.github.io/hit-test/ - please
let me know what you think & file issues in the repo
<https://github.com/immersive-web/hit-test/> if you have any comments. The
hit test for transient input source is not yet covered by the initial
version - this is something that I'm going to work on now.

Additionally, I have a prototype implementation of hit test API (both
regular and for transient input) working in Chrome Canary for Android
behind the flags (instructions below) so if anyone's interested at seeing
how it behaves, take it for a spin and let me know if you have any feedback.

Instructions:
1. Download Chrome Canary.
2. Navigate to chrome://flags, enable #webxr-ar-module and #webxr-hit-test
flags & relaunch the browser.
3. Navigate to a website that leverages proposed hit test API, for example:
https://storage.googleapis.com/chromium-webxr-test/latest.html?target=proposals/phone-ar-hit-test.html

Thanks,
-Piotr