Auto merge of #11766 - emilio:keyframes-parsing, r=SimonSapin,pcwalton

Add `@keyframes` and `animation-*` support.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] There are tests for these changes

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This adds support for parsing `@keyframes` rules, and animation properties. Stylo will need it sometime soonish, plus I want to make animations work in Servo.

The remaining part is doin the math and trigger the animations correctly from servo. I don't expect it to be *that* hard, but probaby I'll need to learn a bit more about the current animation infra (e.g. why the heck is the `new_animations_sender` guarded by a `Mutex`?).

I'd expect to land this, since this is already a bunch of work, this is the part exclusively required by stylo (at least if we don't use Servo's machinery), the media query parsing is tested, and the properties land after a flag, but if you prefer to wait until I finish this up it's fine for me too.

r? @SimonSapin

cc @pcwalton @bholley

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