Add formal constant for voxel extension

[j9liu]

Description

Depends on #1188, so merge that first.

Supports CesiumGS/cesium-unreal#1685. Per PR feedback, the special mode constant used by this extension should be formally defined in cesium-native for use in a runtime.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

N/A, not really testable.

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[david-lively]

LGTM. Waiting on #1188 to merge before merging this one.