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/core.el

@@ -61,6 +61,13 @@ envvar will enable this at startup.")
 (define-error 'doom-private-error "Error in private config" 'doom-error)
 (define-error 'doom-package-error "Error with packages" 'doom-error)
 
+;;; Declare a psuedo theme to store faces and variables in, with no risk of it
+;;; getting saved to `custom-file', or accidentally overwritten by user config.
+(custom-declare-theme 'doom nil)
+(enable-theme 'doom)
+;; But immediately hide it, because it's not a real theme.
+(setq custom-enabled-themes (remq 'doom custom-enabled-themes))
+
 
 ;;
 ;;; Directory variables