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/editor/evil/autoload/ex.el

@@ -165,8 +165,7 @@ buffers."
   "Look up documentation for QUERY.
 
 If QUERY is in the format of an ex command, it will map it to the underlying
-function and open its documentation with `helpful-function'. Otherwise, it will
-search for it with `apropos'.
+function and open its documentation.
 
 If QUERY is empty, this runs the equivalent of 'M-x apropos'. If BANG is
 non-nil, a search is preformed against Doom's manual (with
@@ -180,8 +179,9 @@ non-nil, a search is preformed against Doom's manual (with
               (or (command-remapping #'apropos)
                   #'apropos)))
             ((string-match "^ *:\\([^ ]+\\)$" query)
-             (helpful-function
-              (evil-ex-completed-binding (match-string 1 query))))
+             (funcall (or (command-remapping #'describe-function)
+                          #'describe-function)
+                      (evil-ex-completed-binding (match-string 1 query))))
             ((message "Searching for %S, this may take a while..." query)
              (apropos query t))))))