feat: Close PR for untracked branch

### 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)
```