WebXR Anchors Module - spec draft feedback

Hi all,

Quick reminder to take a look at the current draft of the WebXR Anchors
Module, which can be found here: https://immersive-web.github.io/anchors/.

There is one contentious issue
<https://github.com/immersive-web/anchors/issues/37> on which I'd love to
get some feedback from folks familiar with other AR systems (HoloLens,
ARKit, Magic Leap, etc.) so that we make sure the current approach is not
too ARCore-centric and future-proof - please take a look at it and let me
know what you think.

Apart from that one issue, I think it's in the shape where I could start
kicking off an Intent to Ship blink process, so please also find some time
to review the spec draft in its entirety and let me know if there may be
other issues that I should address.

Thanks,
-Piotr Bialecki