A modification that holds and executes a [SkeletonModificationStack3D].
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<description>
This [SkeletonModification3D] holds a reference to a [SkeletonModificationStack3D], allowing you to use multiple modification stacks on a single [Skeleton3D].
[b]Note:[/b] The modifications in the held [SkeletonModificationStack3D] will only be executed if their execution mode matches the execution mode of the SkeletonModification3DStackHolder.
Sets the [SkeletonModificationStack3D] that this modification is holding. This modification stack will then be executed when this modification is executed.