Revamp XML-Conduit-Stylist API to avoid hard-dependency on Haskell Stylist.

Add utils for implementing :target-within, ensure unlayered styles take precedance.

Fix @layer to play nicely with the conditional at-rules.

If you're not using that, its still integrated into core.
Since the @layer rules are stripped by ConditionalStyles this shouldn't make a difference.

Extract parser, datatypes, & traits into separate decoupling hackage.