[WebXR] Add Teleportation Anchors

[RaananW]

Using (babylon-based) teleportation anchors (or hot-spots?) will allow a developer to define specific points where the user can land within the scene.

As opposed to the current teleportation, which allows the user to land anywhere a floor-mesh is defined, the anchors will limit the user to a specific area, which can be a major benefit in specific cases.