add firefox extension template

externs/webextension_polyfill/urlbar/OnEngagementEvent.hx

@@ -0,0 +1,17 @@
+package webextension_polyfill.urlbar;
+
+/**
+	This event is fired when the user starts and ends an engagement with the urlbar.
+**/
+typedef OnEngagementEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(state:EngagementState) -> Void, providerName:String):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(state:EngagementState) -> Void):Void;
+	function hasListener(callback:(state:EngagementState) -> Void):Bool;
+	function hasListeners():Bool;
+};