Add hook for callers to run their own tests against attributes.

Add new style tree traversals.

Expand :is/:where lowering.

Codestyle: remove unnecessary parens.

Move :root handling into interpretor.

Test counters() handling against longhand rather than shorthand.

Because that's where it'll be passed

Start testing counters implementations, fix syntactic shorthand lowering for counter[s]() functions.

Release new Haskell Stylist & XML Conduit Stylist!

XML Conduit Stylist changed to abstract away Haskell Stylist's breaking datastructure changes.

Correct pseudoclass to pseudoelement rewrites to apply to only rightmost selector.

I previously got the semantics of my datastructures backwards: the head is the rightmost selector.

Remove preceding whitespace from all CSS at-rules.

Lower psuedoclasses & namespaces throughout CSS selectors.

Previously I just applied to them to the rightmost.

Rewrite parsed namespaces according to @namespace rules.

Evaluate attribute namespace selectors.

Parse namespace selectors.

Rewrite trivial cases of :where & :is.

This is mostly for the psuedoselector rewrites performed in Data.CSS.Preprocessor.PsuedoClasses

Compute correct specificity for :is() & :not() pseudoclasses.

Implement :nth-child() & :nth-of-type().

I don't think I'll implement :nth-last-child() & :nth-last-of-type() until I'm shown their value,
Haskell Stylist doesn't yet have access to the data to support them.

Implement :is, :where, & :not functional pseudoclasses.

This is needed by some of the lowerings for other pseudoclasses.
TODO correctly compute specificity.

Prepare to interpret functional pseudoclasses.

Correct legacy :before/:after syntax.