Add WIP compare slider, UnityGltf build, and threejs viewer

[hybridherbst]

This is a WIP but feels like a good starting point. Might make sense pulling this into a branch for now and not on master? UnityGltf build is currently giant because it also still contains glTFast.

[atteneder]

Hi,

Finally found time to look at it. That is, I want to improve the current main version and give this a review before diverging. I love the idea!

What I think we should do before merging into main:

• Broad browser support. Namely Firefox and Safari fail due to missing importmap support
• Default should be to load just one viewer with buttons to add more
• The load menu and drag'n'drop should load in all viewers in sync
• Maintenance strategy. Who keeps all viewer builds up to date? Without proper automation this will become hard to maintain.

Nice to haves:

• Customization via URL parameters (different default viewser, multiple viewers right away, etc.)
• Camera position sync

I like your suggestion of a dedicated dev branch.

Thanks a ton!

[CLAassistant]

All committers have signed the CLA.