Allow renaming a stack branch without rebasing

[GigiLoria5]

gh stack modify currently requires the stack to be rebased before making any changes, including renaming a branch.

There are cases where I want to rename a branch now but postpone rebasing because I expect conflicts and am not ready to resolve them yet.

Could rename-only changes be allowed without normalising the stack first, either through gh stack modify or a dedicated command?

This is related to the broader branch-renaming discussion in #227