This is a first step towards implementing the API, getting all the
boilerplate out of the way. The interfaces are not functional yet,
because I want to keep the pull requests incremental and easy to review.
My branch which contains all the work so far is at
https://github.com/simonwuelker/servo/tree/web-animations.
Testing: These changes are behind a preference that is not enabled in CI
yet. There's also no point in doing so, because there is no
functionality to test.
Part of servo#36950
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>