add firefox extension template

externs/webextension_polyfill/networkstatus/Static.hx

@@ -0,0 +1,12 @@
+package webextension_polyfill.networkstatus;
+
+typedef Static = {
+	/**
+		Returns the $(ref:NetworkLinkInfo} of the current network connection.
+	**/
+	function getLinkInfo():Void;
+	/**
+		Fired when the network connection state changes.
+	**/
+	var onConnectionChanged : webextension_polyfill.events.Event<(details:NetworkLinkInfo) -> Void>;
+};