💥 Replace core-popup with new feature/popup module

This is a breaking change! Update your :popup settings. Old ones will
throw errors!

Doom's new popup management system casts off its shackles (hur hur) and
replaces them with the monster that is `display-buffer-alist`, and
window parameters.

However, this is highly experimental! Expect edge cases.  Particularly
with org-mode and magit (or anything that does its own window
management).

Relevant to #261, #263, #325

init.example.el

@@ -30,6 +30,7 @@
 (require 'core (concat user-emacs-directory "core/core"))
 
 (doom! :feature
+       popup             ; tame sudden yet inevitable temporary windows
       ;debugger          ; FIXME stepping through code, to help you add bugs
        eval              ; run code, run (also, repls)
        evil              ; come to the dark side, we have cookies