allow prefix commands inside nu-prompts

[pyluyten]

nu-prompt-for-keymap expects one key to either be a keybinding or an exit key. This should be fixed: while is fine to quit only when no function can be found - in order not to have to assign q to each keymap! - , oto side prefix keys might be allowed, especially in order to make nu-prompt-for-major-mode work more pleasantly (than, just offering a description...)