Expose get_skeleton() from SkeletonModifier3D

[TokageItLab]

Follow up #91507.

To create a custom SkeletonModifier, a method to safely retrieve the Skeleton was lacked for gdscript and extention.

[lyuma]

Seems ok technically.

Get parent [Skeleton3D] node if found.

Based on this documentation, I do not understand why this method is different from get_parent() as Skeleton3D - I don't like having two methods that do the same thing.

Is if we eventually allow descendants of Skeleton3D to be modifiers then this could be useful separate from get_parent

[TokageItLab]

This method is a kind of remnant from the old api when SkeletonModifier3D could not be a child of a Skeleton, but it is useful for safety and readability.

Based on this documentation, I do not understand why this method is different from get_parent() as Skeleton3D - I don't like having two methods that do the same thing.

In practice, it is not equivalent. It retrieve skeleton from cached ObjectID which is found/stored when parenting. But indeed, for performance reasons, it may be better to cache the pointer or make a wrapper function of get_parent() with some flag of the availavility like:

bool is_child_of_skeleton = true;

Skeleton3D *SkeletonModifier3D::get_skeleton() const {
  if (is_child_of_skeleton) {
    return get_parent();
  }
  return nullptr;
}

Well, for now, I would like to expose it as an API to simplify the code during the development of SkeletonModifier3D. I think we can send a follow-up later to improve the performance.

[akien-mga]

Thanks!