add firefox extension template

externs/webextension_polyfill/events/Event.hx

@@ -0,0 +1,17 @@
+package webextension_polyfill.events;
+
+/**
+	An object which allows the addition and removal of listeners for a Chrome event.
+**/
+typedef Event<T> = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:T, params:haxe.extern.Rest<Dynamic>):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:T):Void;
+	function hasListener(callback:T):Bool;
+	function hasListeners():Bool;
+};