refactor: move doom-highlight-non-default-indentation-h out of core

This functionality falls under the umbrella of "better defaults" rather
than essential defaults, and so belongs in :config default.

Whats more, the naming convention (+emacs-...) was intentional, because
this function (and the :config default module) will be moved to :doom
emacs in the near future.

This also indirectly fixes #8373.

Fix: #8373

modules/config/default/config.el

@@ -83,6 +83,31 @@
   (setq tramp-default-method "ssh")) ; faster than the default scp
 
 
+;;;###package whitespace
+(add-hook! 'after-change-major-mode-hook :append
+  (defun +emacs-highlight-non-default-indentation-h ()
+    "Highlight whitespace at odds with `indent-tabs-mode'.
+That is, highlight tabs if `indent-tabs-mode' is `nil', and highlight spaces at
+the beginnings of lines if `indent-tabs-mode' is `t'. The purpose is to make
+incorrect indentation in the current buffer obvious to you.
+
+Does nothing if `whitespace-mode' or `global-whitespace-mode' is already active
+or if the current buffer is read-only or not file-visiting."
+    (unless (or (eq major-mode 'fundamental-mode)
+                (bound-and-true-p global-whitespace-mode)
+                (null buffer-file-name)
+                buffer-read-only)
+      (require 'whitespace)
+      (set (make-local-variable 'whitespace-style)
+           (cl-union (if indent-tabs-mode
+                         '(indentation)
+                       '(tabs tab-mark))
+                     (when whitespace-mode
+                       (remq 'face whitespace-active-style))))
+      (cl-pushnew 'face whitespace-style) ; must be first
+      (whitespace-mode +1))))
+
+
 ;;
 ;;; Smartparens config