Add KHR_materials_fuzz

[MiiBond]

This extension provides an OpenPBR-compatible fuzz/sheen extension.
The main differences are:

1. The fuzz sits on top of the coat (sheen sat below it)
2. The fuzz has a dedicated weighting factor, whereas sheen only had color. A consequence of this is that fuzz can be darker than the surface below it. This allows effects like black soot.