Now the Model type is usable by class that extends Viewer

[alexandremottet]

Before
If you extend Viewer and call _loadModel you cannot use the type Model anywhere in the extended class.

Now
You can use the type Model

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
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Test environment:
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16118/merge/index.html

To test a playground add it to the URL, for example:

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[bjsplat]

WebGL2 visualization test reporter:

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[bjsplat]

Visualization tests for WebGPU

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