lib: add plugin and theme subcommands and fix `omz pr clean`

- Add plugin and theme subcommand

  Fixes #8961

- Add confirmation prompt to `omz pr clean` command

- Correct behavior on invalid answers on confirmation prompts

  If an invalid option is supplied, don't proceed with the potentially
  destructive action.

Co-authored-by: Rishabh Bohra <rishabhbohra01@gmail.com>