Add distanceDisplayCondition to ModelExperimental

[j9liu]

This PR adds distanceDisplayCondition to ModelExperimental. Pretty straightforward, just mimics the implementation in Model.js. Here's the Sandcastle for testing.

I ended up reorganizing the documentation for the constructor / fromGltf function, but it was to match the order of the parameters in Model. This way it's easier to see the differences between the APIs.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

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

[ptrgags]

@j9liu looks good! I had one small suggestion about variable naming before I merge this.

[j9liu]

@ptrgags updated!

[ptrgags]

Looks good, thanks @j9liu!