Auto merge of #14433 - canaltinova:medialist, r=Manishearth

Implement MediaList interface

<!-- Please describe your changes on the following line: -->
Added MediaList interface and implemented `mediaText`, `length` and `index` attributes.
r? @Manishearth

---
<!-- 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

<!-- Either: -->
- [x] There are tests for these changes

<!-- 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/14433)
<!-- Reviewable:end -->

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

@@ -140,6 +140,7 @@ test_interfaces([
   "KeyboardEvent",
   "Location",
   "MediaError",
+  "MediaList",
   "MediaQueryList",
   "MessageEvent",
   "MimeType",