Formalize workflow for applying EXT_skeleton_humanoid animations onto one or more existing VRM 0.0/1.0 models

[lyuma]

We met at omigroup/gltf-extensions#138

I think there is significant interest in applying this format to existing VRM content

We have actually discussed this as a working group before here: omigroup/gltf-extensions#44 - we didn't make progress because of limited time and effort, so we are grateful for the possibility that this project could provide the answer to this problem.

(apologies for hitting shift-enter prematurely)