fix: map!: allow :map values to be interpolated

With the comma syntax. E.g.

  (let ((maps '(some-mode-map another-mode-map)))
    (map! :map ,maps))

This technique was used in the recent rewrite of the Racket
module (1baebda), but I forgot to include this change, so Racket users
have probably noticed some missing keybinds! This fixes that too.

Amend: 1baebda
Ref: doomemacs#7543

Author: hlissner

lisp/doom-keybinds.el

@@ -307,7 +307,7 @@ For example, :nvi will map to (list 'normal 'visual 'insert). See
                  (:desc
                   (setq desc (pop rest)))
                  (:map
-                  (doom--map-set :keymaps `(quote ,(ensure-list (pop rest)))))
+                  (doom--map-set :keymaps `(backquote ,(ensure-list (pop rest)))))
                  (:mode
                   (push (cl-loop for m in (ensure-list (pop rest))
                                  collect (intern (concat (symbol-name m) "-map")))