📛 Export/Import Type Tagged .morphtarget GLTF

[effs]

Minimal, unstructured scaffolding to import/export type-tagged GLTFs. Since this is more involved than is ideal, export is just supported for .morphtarget (so that it is distinct from a .mesh export.)

You can, however, use file naming to avoid having to select the import format for GLTFs.

BUT

Rather than try to surgically fix things here and there, I think the best option is to:

1. Get rid of the separate Mesh, WithRig and Multimesh exports and just treat everything as cases of Multimesh in a single code path (which is what import does on the return trip.) This also needs a better UI, but should work transparently anyway. -> Doing this in ♻️ Experimental Unified Mesh Exporter #1197

2. Try to separate import/export data handling from the filesystem ops, either by returning data streams from the exporters, or by passing in a fs context that cannot be changed downstream.

3. Then maybe doing something more.

[rfuzzo]

Thanks, looks good (can't comment on the actual mesh logic though).

In the future, it would be great if commits did not contain any emojis please!