Add `Selection.rotationPivot` to `Selection` API

Did other users suggest similar ideas?

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Is your feature request related to a problem? Please describe. I'm working on a plugin, and I realised it would be convenient to reuse the position of the Rotation Pivot for a Selection as the centerpoint/rotation point for this plugin. However, there seems to be no way to retrieve the sprite position or relative selection position of the Rotation Pivot in the API.

Describe the solution you'd like Add Selection.rotationPivot to the API, which would return the position of the rotation Pivot of the given selection. This should probably only work for app.sprite.selection and when UI is on. It makes most sense if the position is relative to the selection itself.

Describe alternatives you've considered There seem to be no alternatives.