No remove command

[wahlrkr]

Currently there is add which adds a new branch to the stack, but no remove which would remove it. unstack is close, but I only managed to remove my whole stack and needed to start over 😬 This would also allow to "map out" the idea first as a plan more easily

As a workflow:

gh stack init

gh stack add <branch_A>
gh stack add <branch_B>
gh stack add <branch_C>

# oops I actually don't need B

gh stack remove <branch_B>