Improve unicode, emoji, & symbol font support

+ Adds doom-emoji-fallback-font-families
+ Adds doom-symbol-fallback-font-families
+ Gives doom-unicode-font higher precedence (and sets it to nil by
  default, since Emacs defaults to symbola already).
+ Store custom face settings in psuedo theme, rather than crowding the
  user theme (which may be immortalized to custom-file, which we want to
  avoid).
+ Now, after-setting-font-hook is guaranteed to fire after *all* fonts
  are set, giving you a more reliable interface to add your own
  font(set) modifications.

More testing is needed to ensure the patched fonts our ligatures modules
don't conflict with this change.

core/autoload/fonts.el

@@ -73,8 +73,7 @@ See `doom-init-fonts-h'."
   (interactive)
   (when doom-font
     (set-frame-font doom-font t))
-  (doom-init-fonts-h)
-  (mapc #'doom-init-extra-fonts-h (frame-list)))
+  (doom-init-fonts-h))
 
 ;;;###autoload
 (defun doom/increase-font-size (count)
@@ -116,7 +115,8 @@ This uses `doom/increase-font-size' under the hood, and enlargens the font by
   :lighter " BIG"
   :global t
   (unless doom-font
-    (user-error "`doom-font' must be set to a valid font"))
+    (or (setq doom-font (face-attribute 'default :font))
+        (user-error "`doom-font' must be set to a valid font")))
   (if doom-big-font
       (let ((font (if doom-big-font-mode doom-big-font doom-font)))
         (set-frame-font font 'keep-size t)