The spec gives us a hard-coded list of functional pseudo-classes and how
to serialize them - but this list is incomplete and likely to always be
outdated compared to the list of pseudo-classes that exist. So instead,
use the generated metadata we already have to serialize their arguments
based on their type.
This fixes :dir() and :has(), which previously did not serialize their
arguments.
Gets us 26 passes (including 6 from that as-yet-unmerged :dir() test).
Submitted to WPT as https://github.com/web-platform-tests/wpt/pull/52598
but in the meantime here's a local version.
The spec for this isn't super thorough, so the tests are based on how
Chrome and Firefox behave. Specifically, Firefox returns the ltr/rtl
keyword in lowercase but Chrome keeps the original case for it.
We currently fail most of these but that will be fixed in subsequent
commits.
