Add support for XRSystem.offerSession

[De-Panther]

Meta Quest Browser supports WebXR offerSession, which displays Enter XR button in the address bar, making it more obvious that the site can switch to XR mode.

Sample can be found at
https://github.com/cabanier/webxr-samples-1/blob/d7d6050e15e0b06aae1cac452d30eb8c628fdb4e/input-selection-2.html#L95

Can only offer one type of session at a time. So the developer needs to prioritize AR, VR, one of them or none.