Autoload cua-toggle-rectangle-mark (Bug#34947)

* lisp/emulation/cua-base.el (cua-toggle-rectangle-mark): Autoload it.
Library cua-base.el binds cua-toggle-rectangle-mark to a key in the
:set function in (defcustom cua-rectangle-mark-key ...), so it should
ensure that the command is defined.

Copyright-paperwork-exempt: yes

lisp/emulation/cua-base.el

@@ -592,6 +592,7 @@ a cons (TYPE . COLOR), then both properties are affected."
 
 (autoload 'cua-set-rectangle-mark "cua-rect"
   "Start rectangle at mouse click position." t nil)
+(autoload 'cua-toggle-rectangle-mark "cua-rect" nil t)
 
 ;; Stub definitions until it is loaded
 (defvar cua--rectangle)