Hint for VR in AR WebXR sessions.

[AdaRoseCannon]

/facetoface This came up in today's call (4/3/25). There have been experiences in the wild want to run almost entirely in VR then occasionally use the AR pass through features. Currently this results in large power and performance requirements as the AR systems are active but not being utilised. Should we have a new session type or an API to quickly allow switching between AR and VR?