implement gltf and glb parser

[SimonDanisch]

Which should return a mesh which uses MultiMesh and works with Makie.mesh(gltf).

[codecov]

Codecov Report

❌ Patch coverage is 70.38961% with 114 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.74%. Comparing base (202463d) to head (69f9090).

Files with missing lines	Patch %	Lines
src/io/gltf.jl	70.15%	114 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   79.25%   75.74%   -3.51%     
==========================================
  Files          11       12       +1     
  Lines         829     1212     +383     
==========================================
+ Hits          657      918     +261     
- Misses        172      294     +122     

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