Add a way to split the last commit as a separate branch

[draftcode]

Relatively often, I find myself adding a commit and then realize that that commit should be in another branch. It's nice if there's a convenient way to split the last commit as a separate branch.

[Brookke]

I had a similar request in the discord adding that comment here:

av stack squash takes the commits of the current branch and puts them into a single commit
av stack split takes the commits of the current branch and splits them into separate stacked branches.

Perhaps split could take a flag e.g. --last-n-commits=n so that you can just split of the last commit

av stack fold combines the current branch with its parent and updates the children in the stack