add firefox extension template

externs/webextension_polyfill/alarms/Alarm.hx

@@ -0,0 +1,18 @@
+package webextension_polyfill.alarms;
+
+typedef Alarm = {
+	/**
+		Name of this alarm.
+	**/
+	var name : String;
+	/**
+		Time when the alarm is scheduled to fire, in milliseconds past the epoch.
+	**/
+	var scheduledTime : Float;
+	/**
+		When present, signals that the alarm triggers periodically after so many minutes.
+		Optional.
+	**/
+	@:optional
+	var periodInMinutes : Float;
+};