Implement protocol activation

[jdm]

We want to support launching directly into immersive content from other device content (such as Edge). The easiest way to do that is by registering a custom protocol handler and using information passed through it to load a page in Servo and provide implicit permission for entering immersive mode.

[paulrouget]

The protocol handler now works: #24008

We now need:

1. find a way to create and open the servo:// URL from edge
2. figure out a way to trigger the immersive mode in Servo. We can't just guess which JS function to call to initiate the immersive mode.

[paulrouget]

find a way to create and open the servo:// URL from edge

I mean, we need to decide when Edge should re-write the URL with the servo prefix, and how it will do it. Maybe a user script? Not sure what mechanics Edge supports.

[jdm]

Yes, there are some possibilities - one option requires modifying Edge to detect when Firefox Reality is installed, notice when a page contains WebXR usage and popping up a little notification that would launch FxR with the right URL if selected. There is also ongoing spec discussions about how to launch directly into immersive mode - if there were a DOM event which means "now is the right time to launch immersive mode" which content could watch for, we could fire that when the page loads.

[paulrouget]

Relevant: immersive-web/webxr#426