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/config/default/+emacs-bindings.el

@@ -553,9 +553,6 @@
         "<" #'help-go-back
         "n" #'forward-button
         "p" #'backward-button)
-      (:after helpful
-        :map helpful-mode-map
-        "o" #'link-hint-open-link)
       (:after apropos
         :map apropos-mode-map
         "o" #'link-hint-open-link

modules/config/default/+evil-bindings.el

@@ -102,8 +102,6 @@
 
       (:after help :map help-mode-map
        :n "o"       #'link-hint-open-link)
-      (:after helpful :map helpful-mode-map
-       :n "o"       #'link-hint-open-link)
       (:after info :map Info-mode-map
        :n "o"       #'link-hint-open-link)
       (:after apropos :map apropos-mode-map