Improve gltf-io and manifold-gltf

[elalish]

I had noticed some problems with https://manifoldcad.org/make-manifold, where objects with properties were sometimes losing them. It turns out I wasn't reading and writing attributes correctly sometimes, partly because of my gltf-io API.

Here I've:

1. Transitioned gltf-io and manifold-gltf to TS (which caught several errors on its own)
2. Changed the gltf-io API to use an id map and package each material with its attributes as Properties
3. Applied usage groups to remove the gltf-transform warnings
4. Fixed disposal and verified no unused or duplicate entries are being exported to the glTF
5. Simplified the examples and verified that all glTFs now pass validation
6. Added docs to gltf-io
7. Improved Mesh declaration by adding MeshOptions interface.

Ideally I should look into publishing gltf-io and manifold-gltf to npm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7273c9b) 91.41% compared to head (5fe87b2) 91.41%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #612   +/-   ##
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  Coverage   91.41%   91.41%           
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  Files          35       35           
  Lines        4496     4496           
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  Hits         4110     4110           
  Misses        386      386           

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[pca006132]

I am not very familiar with this, so I guess you can just merge it if it works :)