add missing externs to repo

externs/webextension_polyfill/userscripts/RegisteredUserScript.hx

@@ -0,0 +1,11 @@
+package webextension_polyfill.userscripts;
+
+/**
+	An object that represents a user script registered programmatically
+**/
+typedef RegisteredUserScript = {
+	/**
+		Unregister a user script registered programmatically
+	**/
+	function unregister():Void;
+};