Import ActionController::MissingRenderer from Rails

[zzak]

Happened upon this exception when patching this file in rails/rails#48327, the documentation references a method which was moved to this gem, and nothing else in Rails uses it internally.

I think we can move it here and deprecate it from Rails, which I've started working on in rails/rails#48328

[carlosantoniodasilva]

@zzak I can get this into responders and released, but we'll have to update the Rails side of things a bit apparently... we might also have to detect here if the constant is not already defined or something (suppressing possible warnings), I think?

[zzak]

@carlosantoniodasilva Thanks for the ping, yeah I'm not sure but maybe we can do unless defined??

I have to fix the tests (and probably rebase) upstream too, but I think the right thing to do is do the check on responders side.