tweak: helpful-set-variable-function: use setq!

helpful-set-variable-function defaults to setopt if it's available, setq
otherwise. `setopt` is superior for this use case because it uses
custom.el's mechanism for setting variables (thus respecting custom.el's
setters and type checks), however, it is unavailable in Emacs 28 and
earlier (`setopt` was introduced in 29).

Doom defines `setq!`, which is a drop-in replacement for setopt, and is
functional in Emacs 28, so it is the better option here.

Ref: #7527

lisp/doom-editor.el

@@ -584,7 +584,9 @@ current buffer."
                    filename))
           (prog1 (apply fn args)
             (when (buffer-live-p buf)
-              (with-current-buffer buf (goto-char pos)))))))))
+              (with-current-buffer buf (goto-char pos))))))))
+  :config
+  (setq helpful-set-variable-function #'setq!))
 
 
 (use-package! smartparens