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/lang/emacs-lisp/autoload.el

@@ -116,8 +116,14 @@ if it's callable, `apropos' otherwise."
                            '(outline org-fold-outline))
                  (org-show-hidden-entry))))
            'deferred))
-        (thing (helpful-symbol (intern thing)))
-        ((call-interactively #'helpful-at-point))))
+        (thing
+         (funcall (or (command-remapping #'describe-symbol)
+                      #'describe-symbol)
+                  (intern thing)))
+        ((call-interactively
+          (if (fboundp #'helpful-at-point)
+              #'helpful-at-point
+            #'describe-symbol)))))
 
 ;; DEPRECATED Remove when 28 support is dropped.
 (unless (fboundp 'lisp--local-defform-body-p)