add firefox extension template

externs/webextension_polyfill/history/VisitItem.hx

@@ -0,0 +1,29 @@
+package webextension_polyfill.history;
+
+/**
+	An object encapsulating one visit to a URL.
+**/
+typedef VisitItem = {
+	/**
+		The unique identifier for the item.
+	**/
+	var id : String;
+	/**
+		The unique identifier for this visit.
+	**/
+	var visitId : String;
+	/**
+		When this visit occurred, represented in milliseconds since the epoch.
+		Optional.
+	**/
+	@:optional
+	var visitTime : Float;
+	/**
+		The visit ID of the referrer.
+	**/
+	var referringVisitId : String;
+	/**
+		The $(topic:transition-types)[transition type] for this visit from its referrer.
+	**/
+	var transition : TransitionType;
+};