org-set-font-lock-defaults: Fix checkbox fontification

* lisp/org.el (org-set-font-lock-defaults): Use full
`org-list-full-item-re' that accounts for alphabetical bullets as
well.

Reported-by: pondersson <pondersson@gmail.com>
Link: https://orgmode.org/list/CAHewz5N5vNMiWFuG3t=bZoyVmsYswyerZ6VsLC88DGTXR+AhOA@mail.gmail.com

lisp/org.el

@@ -5849,8 +5849,7 @@ needs to be inserted at a specific position in the font-lock sequence.")
 	  ;; Emphasis
 	  (when org-fontify-emphasized-text '(org-do-emphasis-faces))
 	  ;; Checkboxes
-	  '("^[ \t]*\\(?:[-+*]\\|[0-9]+[.)]\\)[ \t]+\\(?:\\[@\\(?:start:\\)?[0-9]+\\][ \t]*\\)?\\(\\[[- X]\\]\\)"
-	    1 'org-checkbox prepend)
+	  `(,org-list-full-item-re 3 'org-checkbox prepend lax)
 	  (when (cdr (assq 'checkbox org-list-automatic-rules))
 	    '("\\[\\([0-9]*%\\)\\]\\|\\[\\([0-9]*\\)/\\([0-9]*\\)\\]"
 	      (0 (org-get-checkbox-statistics-face) prepend)))