WebXR - check if app is running AR or VR

[Mugen87]

@julapy

ive been finding the distinction between AR and VR in terms of XR in threejs confusing.

The ARButton and VRButton concept is just a thin layer on top of WebXR which distinguishes between AR and VR. So when requesting a XR session, you have to decide between immersive-vr or immersive-ar.