Screen-based AR support

[hybridherbst]

I understand this may not be a priority for Meta, but I wanted to capture my thoughts here nonetheless. We're using the simulator for AR cases as well, including phones.

A couple of behaviours differ right now that could be improved:

• DOMOverlay element isn't styled as it should, should be using styles like this to ensure it fills the screen
• Clicks / touches should simulate controllers in targetRayMode screen

And some more of lower importance:

• leaving AR sessions ends up in a bad state (this is just the case with the simulator right now but could be better I think)
• ability to choose the environment blend mode and simulation for those modes