Stylo: Add support for -moz-* pseudo-classes for alt text
<!-- Please describe your changes on the following line: -->
---
<!-- 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
- [X] These changes fix [Bug 1341642](https://bugzilla.mozilla.org/show_bug.cgi?id=1341642)
<!-- Either: -->
- [X] These changes do not require tests because they are stylo 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/16028)
<!-- Reviewable:end -->
a2c33ac6e6
