feat(submit): Prompt to Assign PR to Current User

[refcell]

Description

Add a prompt when running st submit whether to assign the current user to the PR.

st should remember this option in the global config, so it doesn't ask again if the user choses no assignee the first time.
If the user choses to assign themselves, st can prompt y/N with that user in the global config.
Maybe expose a way to set this default assignee in the global config? e.g. st set default.assignee <username>.