Add 3D Model Viewer extension

[imboni]

Summary

• Adds 3d-model-viewer to the Zed extensions index.
• Registers https://github.com/imboni/zed-gltf-model-viewer.git as a submodule at extensions/3d-model-viewer.
• Points the extension entry at version 0.1.0.

Notes

The extension is named 3D Model Viewer because it is intended as a general 3D model previewer. The current implementation supports .glb and .gltf files.

Validation

• cargo check --locked
• npm run build
• COREPACK_ENABLE_PROJECT_SPEC=0 pnpm sort-extensions
• COREPACK_ENABLE_PROJECT_SPEC=0 pnpm build
• COREPACK_ENABLE_PROJECT_SPEC=0 pnpm test
• git diff --cached --check
• Official extension index validation via validateExtensionsToml, validateGitmodules, and validateGitmodulesLocations.

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