Auto merge of #12467 - jeenalee:jeena-headersAPI, r=jdm

Add the append method for the Headers API

<!-- Please describe your changes on the following line: -->
This commit adds the append method for the Headers API. @malisas and I are both contributors.

There are a few TODOs related:
- The script needs to parse the header value for certain header names to decide the header group it belongs
- There are possible spec bugs that could change what a valid header value looks like (related: [issue page](https://github.com/whatwg/fetch/issues/332))

There are WPT tests already written for the Headers API, but they will fail as the Headers API is not fully implemented.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because tests for the Headers API already exists, but this commit does not implement the interface fully. The tests will fail.

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12467)
<!-- Reviewable:end -->

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>;
+ * }; */