DOM Overlay status update

I've incorporated feedback into the proposed DOM Overlay spec at
https://immersive-web.github.io/dom-overlays/ , and did a pass through the
issues and PRs in the repository at
https://github.com/immersive-web/dom-overlays . Thank you to everyone who
had been providing feedback!

If you had been waiting for things to stabilize, I think now would be a
good time to take a look at it, and please file issues and/or contact me if
you have questions about it.

If you want to try it, there's a sample at
https://storage.googleapis.com/chromium-webxr-test/latest.html?target=proposals/phone-ar-hit-test.html?debugSources=true%26iframe=true
which
works in current Chrome Canary with "WebXR Incubations" enabled in
chrome://flags. This sample includes a visualization of WebXR input events
and how they interact with DOM events.

(The current Chrome Canary implementation doesn't include some of the
recent spec changes yet that were made based on feedback, but this
essentially just affects naming for the CSS pseudoclass and integration
with Fullscreen API and should be resolved shortly, and the changes don't
affect this sample page.)