refactor!: move helpful from :core to :lang emacs-lisp

BREAKING CHANGE: This moves helpful.el out of core into :lang
emacs-lisp. Since most (all) people have this module enabled, this
shouldn't make a difference for most people, but if you're one of the
few that don't have :lang emacs-lisp enabled, Doom will revert to using
Emacs' built-in help.el and describe-* commands.

Others can also disable helpful with (package! helpful :disable t) if
they prefer Emacs' built-in help system, which wasn't possible before,
because it was a core package.

This was done as part of an ongoing effort to slim down Doom's core in
preparation for v3.

modules/completion/ivy/config.el

@@ -226,11 +226,6 @@ workable results ripgrep produces, despite the error."
                (if (= code 2) 0 code)))
       (apply fn args)))
 
-  ;; Integrate with `helpful'
-  (setq counsel-describe-function-function #'helpful-callable
-        counsel-describe-variable-function #'helpful-variable
-        counsel-descbinds-function #'helpful-callable)
-
   ;; Decorate `doom/help-custom-variable' results the same way as
   ;; `counsel-describe-variable' (adds value and docstring columns).
   (ivy-configure 'doom/help-custom-variable :parent 'counsel-describe-variable)