Determine ultimate `Transforms` API after splitting into more specific classes

As part of https://github.com/CesiumGS/cesium/pull/13759, the Transforms class is being split into two more focused and cohesive classes:

• FixedFrameTransforms: all the ENU transforms, and others that can be computed via direct Matrix and Cartesian math.
• CelestialFrameTransforms: Transforms that depend on time-dependent data describing relative positions of celestial bodies. This class, because of the data requirement, has a wider range of dependencies including Resource.

Within #13759, both smaller classes are marked @private, and all the members are re-exported from a combined Transforms class, so that no change is made to the public API.

However, the smaller classes might actually make a cleaner and more intuitive public API. Some options:

• Expose the two new classes as public, in parallel with Transforms for perpetual backward compatibility
• Deprecate Transforms and encourage users to switch to the new classes. Given the extensive use of Transforms, including many published examples and tutorials, the deprecation period would need to be fairly long.