oc: Allow citations following an item bullet

* lisp/oc.el (org-cite--allowed-p): Expand the allowed cases to include
"item", which lets one insert a citation following a bullet, e.g. "- ".
We just have to check to make sure the citation actually is at least one
character away from the bullet, or five if a checkbox is present.

lisp/oc.el

@@ -1420,6 +1420,12 @@ ARG is the prefix argument received when calling `org-open-at-point', or nil."
 			  (skip-chars-backward " \r\t\n")
 			  (if (eq (org-element-class context) 'object) (point)
 			    (line-beginning-position 2)))))
+     ;; At the start of a list item is fine, as long as the bullet is unaffected.
+     ((eq type 'item)
+      (> (point) (+ (org-element-property :begin context)
+                    (org-get-indentation)
+                    (if (org-element-property :checkbox context)
+                        5 1))))
      ;; Other elements are invalid.
      ((eq (org-element-class context) 'element) nil)
      ;; Just before object is fine.