Export IGLTF and support scene plugin

[zhuxudong]

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• Tests for the changes have been added (for bug fixes / features)
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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (89a417f) 65.07% compared to head (7617b53) 65.02%.

❗ Current head 7617b53 differs from pull request most recent head 058f525. Consider uploading reports for the commit 058f525 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1816      +/-   ##
==========================================
- Coverage   65.07%   65.02%   -0.05%     
==========================================
  Files         473      473              
  Lines       23771    23773       +2     
  Branches     3369     3369              
==========================================
- Hits        15468    15459       -9     
- Misses       7103     7113      +10     
- Partials     1200     1201       +1     

Files	Coverage Δ
packages/loader/src/GLTFLoader.ts	100.00% <100.00%> (+14.28%)	⬆️
.../src/gltf/extensions/KHR_draco_mesh_compression.ts	4.67% <ø> (ø)
packages/loader/src/gltf/parser/GLTFMeshParser.ts	69.71% <ø> (ø)
...ckages/loader/src/gltf/parser/GLTFParserContext.ts	95.16% <100.00%> (ø)
packages/loader/src/gltf/parser/GLTFSceneParser.ts	57.93% <100.00%> (+0.58%)	⬆️

... and 3 files with indirect coverage changes

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