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.

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tests/wpt/mozilla/tests/mozilla/interfaces.html

@@ -48,6 +48,7 @@ test_interfaces([
   "FocusEvent",
   "FormData",
   "HashChangeEvent",
+  "Headers",
   "HTMLAnchorElement",
   "HTMLAppletElement",
   "HTMLAreaElement",

tests/wpt/mozilla/tests/mozilla/interfaces.worker.js

@@ -44,6 +44,7 @@ test_interfaces([
   "FocusEvent",
   "FormData",
   "HashChangeEvent",
+  "Headers",  
   "HTMLAnchorElement",
   "HTMLAppletElement",
   "HTMLAreaElement",