Fix #3650: void-variable tab-prefix-map

Seems like some OSes' package managers are serving up a build of Emacs
where it doesn't define `tab-prefix-map`, but uses it (or another
package uses it--honestly, it difficult to tell. I can't reproduce this,
but I've gotten reports of it on Arch and Ubuntu).

It'll likely go away on its own, so I'll remove this after a year or
something.

core/core.el

@@ -48,6 +48,12 @@
     (setq file-name-handler-alist doom--initial-file-name-handler-alist))
   (add-hook 'emacs-startup-hook #'doom-reset-file-handler-alist-h))
 
+;; REVIEW Fixes 'void-variable tab-prefix-map' errors caused by packages that
+;;        prematurely use this variable before it was introduced. Remove this in
+;;        a year.
+(unless (boundp 'tab-prefix-map)
+  (defvar tab-prefix-map (make-sparse-keymap)))
+
 ;; Just the bare necessities
 (require 'subr-x)
 (require 'cl-lib)