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
