Mesh shaders support (meshlets)

I would propose mesh shaders support for GLTF, which uses meshlets (256 triangles per triangle) i.e. I suggest chunked or fragmented mesh support.

The proposal concerns adding mesh shader and meshlet support to glTF, but names no files, tests, or implementation entry point. First clarify the intended glTF representation and compatibility requirements for meshlets, then identify the relevant specification and validation areas. Done means an agreed design and corresponding specification, validation, and test changes.

Written by the indexing model from the issue text.