[UX] Inject "mini modules forms" in various places where it makes sense

I thought of this idea while filing #5948.

It would be cool if we could inject a "mini" modules form in the "Regional and language" page in admin/config/regional, so that people can enable these modules without having to navigate elsewhere in the admin UI and then return back to the page where they were expecting to find the relevant options. Kinda what we are doing with the permissions matrices we are injecting in other places.

I have a few ideas about the UI, but if people believe that the interface could get overwhelming and cluttered, then we could hide these options in a collapsed fieldset. ...OR simply stick with the help text and link from #5948 and shoot this request down.

In general though, perhaps we should be shifting from the developer-centric notion of "modules" into features or functionality instead. The modules listing page and the project browser are cool and we should still keep them, but what I am trying to explore is a way for some modules to be options in various places where it makes sense from a non-Drupal/non-Backdrop/non-developer person.