fix broken QF cancellation from editor instance (#5054)

Define missing cancel method on io object passed from the
editor instance to quickFormat dialog. Without this
Escape keypresses throws an error because io.cancel is
undefined.

Followup to #4859

chrome/content/zotero/xpcom/editorInstance.js

@@ -1149,6 +1149,14 @@ class EditorInstance {
 				}
 			},
 
+			/**
+			 * Cancel changes to the citation
+			 */
+			cancel: function () {
+				this.citation.citationItems = [];
+				return this.accept();
+			},
+
 			/**
 			 * Get a list of items used in the current document
 			 * @return {Promise} A promise resolved by the items