Add the append method for the Headers API for the Fetch API

This commit will add the append method and associated helper functions, and introduce any necessary changes for it.

components/script/dom/webidls/Headers.webidl

@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://fetch.spec.whatwg.org/#headers-class
+
+/* typedef (Headers or sequence<sequence<ByteString>>) HeadersInit; */
+
+/* [Constructor(optional HeadersInit init),
+ * Exposed=(Window,Worker)] */
+
+interface Headers {
+  [Throws]
+  void append(ByteString name, ByteString value);
+};
+
+/* void delete(ByteString name);
+ * ByteString? get(ByteString name);
+ * boolean has(ByteString name);
+ * void set(ByteString name, ByteString value);
+ * iterable<ByteString, ByteString>;
+ * }; */