`mr-model`: Add Support for all Models

[michaelthatsit]

Currently we only support STL because it was the first model we needed to load.

adding support for gltf, obj, and usd should be fairly straight forward.

Bonus points if it preloads models and instance meshes any duplicates.

i.e. this should load one model and InstanceMesh any subsequent model

<mr-model src="logo.gltf"></mr-model>
<mr-model src="logo.gltf"></mr-model>
<mr-model src="logo.gltf"></mr-model>