Consider moving 'property-graph' into glTF Transform

[donmccurdy]

I wrote property-graph as the underlying graph data model for glTF Transform. It's an external repository because it's generalized enough for other uses, but in practice I think it's only used by glTF Transform, 5 years in.

For easier maintenance (and a no-runtime-dependency core) I think it could make sense just to move the property-graph codebase into glTF Transform.