Add feature detection to XRSystem

[wmurphyrd]

When designing an Immersive Web page, it would be nice to be able to set expectations for the user about what will happen when they activate a link, i.e. whether they will need to manually re-enter immersive mode when reaching their destination, but that doesn't seem possible with the current proposal nor its existing implementations

Perhaps an extension to navigator.xr could provide information about whether immersive navigation is supported and whether it is limited to same-origin