Cesium3DTileContent child classes should probably not be private

`Cesium3DTileContent` is an abstract public class in the API, instances of which are exposed in `Cesium3DTile.content`. `Cesium3DTileContent` child classes, `X3DTileContent`, are all private. While these classes are generally not constructed directly, the classes should likely be public since they are accessable via `Cesium3DTile.content`.

We noticed this when [deprecating `readyPromise` properties](https://github.com/CesiumGS/cesium/pull/11173) throughout the API. Even though the classes were private, we threw deprecation warnings just in case.

Review the Cesium3DTileContent declaration and its X3DTileContent child classes, then trace how Cesium3DTile.content exposes them. Check the existing API visibility and deprecation handling before making the classes public. Done means the child classes are publicly accessible and the relevant API behavior remains consistent.

Written by the indexing model from the issue text.