index option for adding primitive to PrimitiveCollection

[hpinkos]

I have a use case where I need to add a primitive to the collection at index 0. This mimics what ImageryLayerCollection does.

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ 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.
• Works (or fails gracefully) in IE11.

[likangning93]

@hpinkos just one comment, otherwise looks good:

[hpinkos]

Thanks @likangning93, ready!

[likangning93]

Does this also warrant an update to CHANGES.md?

[hpinkos]

Sure! why not