Allow using SkeletonModification3DJiggle and SkeletonModification2DJiggle without a target.

[CheesecakeCG]

Describe the project you are working on

An animal themed 3D person shooter. Pretty much every character has tails and ears that need this, along with straps and latches and other sci-fi secondary animation accessories.

Describe the problem or limitation you are having in your project

Setting up many bones to have jiggle physics is complicated and time consuming using PhysicalBones.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Allowing using SkeletonModification3DJiggle and SkeletonModification2DJiggle without a target would make it much faster and simpler to create simpler jiggle physics setups without add-ons (and once the UI bugs are fixed, however while writing this I realized that the skeleton modifications stack is going to be removed godotengine/godot#65801, but I'm still submitting this in-case there's a similar replacement.)

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

When the "Target" option is unset, the jiggle bones will use the pose of the bones as if jiggle were not applied as the destination pose for their jiggles.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Yes, but doing so is is not trivial. Jiggle bones really should be something an artist can set up without programming knowledge.

Is there a reason why this should be core and not an add-on in the asset library?

There are already addons for creating jiggle bones that fix this issue, but the built in SkeletonModification*DJiggle is already very close to being able to completely replace these.