add firefox extension template

externs/webextension_polyfill/dns/DNSRecord.hx

@@ -0,0 +1,19 @@
+package webextension_polyfill.dns;
+
+/**
+	An object encapsulating a DNS Record.
+**/
+typedef DNSRecord = {
+	/**
+		The canonical hostname for this record.  this value is empty if the record was not fetched with the 'canonical_name'
+		flag.
+		Optional.
+	**/
+	@:optional
+	var canonicalName : String;
+	/**
+		Record retreived with TRR.
+	**/
+	var isTRR : String;
+	var addresses : Array<String>;
+};