add firefox extension template

externs/webextension_polyfill/identity/AccountInfo.hx

@@ -0,0 +1,11 @@
+package webextension_polyfill.identity;
+
+/**
+	An object encapsulating an OAuth account id.
+**/
+typedef AccountInfo = {
+	/**
+		A unique identifier for the account. This ID will not change for the lifetime of the account.
+	**/
+	var id : String;
+};