Offersession support

[TiborUdvari]

Nature of issue?

• Found a bug
• Existing feature enhancement
• New feature request

Which area does this problem relate to?

• VR
• AR(ARCORE)
• AR(MARKER)

Feature enhancement details:

navigator.xr.offerSession adds a better integrated button to advertise XR capabilities. It currently only works on Meta devices I think. I first saw it in Three JS. I'll open a PR for this one later.

Here is an example implementation:
https://github.com/cabanier/webxr-samples-1/blob/d7d6050e15e0b06aae1cac452d30eb8c628fdb4e/input-selection-2.html#L95

How it looks in the Three JS examples:

I couldn't find any documentation, except the meeting notes and Three JS code. Probably not a good sign.
https://www.w3.org/2023/04/25-immersive-web-minutes.html#t04