Viewer loading progress bar

[ryantrem]

This PR adds a default loading progress bar. The styling can be customized (via CSS variables for things like color or the progress-bar part for things like height, border radius, etc.) or completely replaced with custom UI (via the progress-bar slot). For cases where we don't know the size of the download, it is a simple indeterminate progress bar.

Currently this is only for model source download, it does not take into account environment texture download. I think we should probably bring this in as well, but it requires more changes to core. Open to thoughts though.

Screen.Recording.2024-11-06.164053.mp4

[bjsplat]

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[bjsplat]

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[bjsplat]

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