libraries/prompter: Resurrect return-* API

[aartaka]

We had a nice function in Prompter before atlas-engineer/nyxt#2655: return-selection. It was a nice, actionable, discoverable name, quite rooted in both programmer lingo and the meaning of the library itself.

Now, the idea of "selection" is deprecated for good, but the actionability of "return"-ing something is still useful. What I suggest is:

• Adding a return function to Prompter.
• Move all the channel management operations from run-action-on-return to it.
• Call run-action-on-return there (suddenly, it all makes sense: run-action-on-return is called inside return, which returns the result of prompting!)

Original commentary: atlas-engineer/nyxt#2655 (comment)