Consider exposing Blendshapes

[alcooper91]

@blairmacintyre in immersive-web/administrivia#125 Stated:

One immediate comment is that you should consider including blendshapes with the mesh data. Most face detection platforms either supply them (e.g., ARKit), or if they need to be calculated, are probably best done at the native level. Since a common use is to animate a 3D model, not just draw the mesh, they will be essential.

After a bit of research, while ARKit does expose blendshapes, there is not currently an ARCore API to expose them. We should evaluate how popular/used they would be, and consider adding them. They could be added as an optional enhancement to the FaceMesh type to prevent undue burden if they cannot be calculated. (It's unclear also if Desktop would be able to support them from a single camera).