Adds support for magit-branch-spinoff

[bezbac]

This PR adds support for behavior similar to magit-branch-spinoff.
I wanted the behavior myself, saw that other users requested it in #124, received eight thumbs-up emojis, and it looked reasonably straightforward to implement.

I'm open to feedback and other ideas on improving this.

[glasser]

This isn't the same as what magit-branch-spinoff does. Spinoff doesn't just make-and-checkout a new branch for the current commit: it also rolls the current branch back to upstream. The idea is it lets you do a workflow where you make a commit or two first (on main) before you create the new branch, and then it moves those commits to your new branch and takes them off of main.

The functionality in this PR might also be useful and maybe is a better workflow than spinoff but I'd encourage you to give it a different name (or update this to match what spinoff does in magit).

[bezbac]

@glasser Thanks for clarifying. I haven't used magit itself much, only this VSCode extension. Seems like I missed those details. I will have another look at implementing the automatic moving of the commits. 👍