This is a quick and dirty script that attempts to take the current selection and center it on the current canvas in aseprite.
It is immensely helpful to me, but it hasn't been tested extensively. There are no guarantees. If it doesn't work for you, let me know and I will either try to fix it or offer you any ideas I have on how to fix it.
I'm just getting started with aseprite and this may not be the best way to do it at all... it really speeds up my workflow, though.
License is MIT. Do what you want with it.