Support for outline mode

[ScamChamp]

I found this code snipppet in an old reddit descusion, and it doesn't work any more in treesit-major modes because they do not use outline-regexp.

(defun use-outline-for-imenu ()
  (require 'outline)
  (setq imenu-create-index-function
        'imenu-default-create-index-function
        imenu-generic-expression
        (list (list nil (concat "^\\(?:" outline-regexp "\\).*$") 0))))