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.

Integrate @layer support into core engine!

Tweak priority to be a list for the sake of cascadeLayers.

Rewrite trivial cases of :where & :is.

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

Improve documentation, release version 1.2

Write reference documentation.

Implement var() via a PropertyParser decorator.

Add additional hooks for callers:

* Resolve relative URLs.
* Set priority on any stylesheet implementation.
* Substitute content during XML Conduit traversal in place of computed styles.

Rework psuedoelement infrastructure so they can be their own boxes.

Code cleanliness fixes.

Factorize out cascade logic.

Add method to override CSS inheritance.

Test cascade logic.

Test/fix cascade logic for selector specificity.

Switch to new namespace for consistency with other modules like css-syntax.

Draft alternate API.