Feature request: Simplify branch renaming

[GigiLoria5]

Renaming a stack branch currently requires opening the TUI (via gh stack modify), pressing r, entering the new name, and saving.

It would be useful to support a direct CLI option for this, for example:

gh stack modify --rename-to <new-name>

Obv, the exact syntax could differ, but the goal would be to let users rename the current stack branch without opening the interactive editor.