Support for configuring where to submit via sl pr without changing default

[trevor-inflection]

After reading the documentation I am not sure if this exists, so just asking

Suppose you have a local commit that you want to submit to github via sl pr submit onto a branch with many commits.

By default it creates a PR that merges into main, but is there a way to merge it into a different branch?

If you do sl goto <branch head> sl rebase ... sl pr submit

then it will try to submit the entire stack of commits that you rebased onto.

I know that the documentation says that PRs will be created against default but I think it may be advantageous to add support to make them against an arbitrary remote bookmark.