Ensure spec parity between Model and ModelExperimental

[j9liu]

This PR reorganizes ModelExperimentalSpec, and adds a handful of new specs to ModelExperimental and related classes. These are specs from ModelSpec that didn't have existing ModelExperimental counterparts.

There are two xit-ed unit tests. We need to return to these later and fix them:

• "disables culling"
• "renders model with emissive texture"

For comparison, here's how the latter test looks:

Model	ModelExperimental

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ptrgags]

@j9liu specs are running fine though when I compare the spec lists looks like there's quite a few tests the old Model had that aren't in ModelExperimental. Maybe that was intentional (e.g. some render tests might be better handled in other files) but wasn't sure.

[j9liu]

Two things to note from this PR:

• GltfJsonLoader automatically upgrades glTF 1.0 to glTF 2.0. We should remove that officially when we deprecate glTF 1.0 models.
• Trying to include a check for the glTF magic in GltfLoader broke a lot of unit tests, so I'm not doing anything with that for now.

[j9liu]

@ptrgags updated!

[ptrgags]

@j9liu almost there, just a couple comments!

[j9liu]

@ptrgags updated!

[ptrgags]

Updates look good, thanks @j9liu!