add firefox extension template

externs/webextension_polyfill/action/OnClickData.hx

@@ -0,0 +1,17 @@
+package webextension_polyfill.action;
+
+/**
+	Information sent when a browser action is clicked.
+**/
+typedef OnClickData = {
+	/**
+		An array of keyboard modifiers that were held while the menu item was clicked.
+	**/
+	var modifiers : Array<OnClickDataModifiersItemEnum>;
+	/**
+		An integer value of button by which menu item was clicked.
+		Optional.
+	**/
+	@:optional
+	var button : Float;
+};