VRM file support

[atteneder]

VRM - 3D humanoid avatar file format

In a nutshell this format extends glTF to make it fit for avatar usage.

There's already support for it via UniVRM (not certified), so maybe the best thing to do is...nothing.

But maybe it's mutually beneficial to explore benefits of each implementation. Potential outcomes, not mutually exclusive:

• Use glTFast as glTF backend in UniVRM
• Add VRM support to glTFast (via another ScriptedImporter)
• Create a new VRM package that depends on glTFast as glTF backend
• Harmonize parts of the code bases (e.g. materials/shaders)