Support KHR_mesh_quantization

[sotrh]

I was messing around with gltfpack trying to improve the performance of some of our models, but when trying the load a model that had been optimized, calling read_positions() got a None on the model (the unoptimized model work just fine). After some digging I found that that gltfpack uses the KHR_mesh_quantization extension to optimize how models are store/loaded.

Given that both ThreeJS and BabylonJS support this extension, I don't think it's too big of an ask.

I'm aware of extension_value() so I have a potenial work around, but it still would be a nice feature.

[sotrh]

Closing this in favor of #400