This is pretty much just wiring up the necessary stylo methods. Note
that the `exportparts` attribute is not yet supported, I'll do that in a
followup change
Testing: Covered by existing web platform tests.
This is the first half of https://github.com/servo/servo/issues/35349
Fixes https://github.com/servo/servo/issues/37325
---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
