feat: Close PR for untracked branch

[refcell]

Description

If you have an open stack, and you untrack a branch, a nice feature for st submit would be to ask to close the PR associated with that untracked branch.

Consider

C
B
A
main (trunk)

We untrack branch B and re-submit the restacked stack, the PR from B into A should be closed by st submit so the updated stack looks like:

C
A
main (trunk)