Merge branch 'main' of git.adrian.geek.nz:/srv/git/haskell-stylist into main

Integrate @layer support into core engine!

Parse @layer rules.

Remove Stylist-Traits APIs from Stylist Core, leaving backwards-compatible stubs.

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

Remove preceding whitespace from all CSS at-rules.

Parse namespace selectors.

Improve documentation, release version 1.2

Release Haskell Stylist 1.1.0.0!

Test rules capturing of ConditionalStyleSheet.

Fix precedence of conditional blocks (+API redesign)

Draft interpretor for @supports.

Implement @document rules (save regexp() conditions).

I've still got to select a regexp library to power it.

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.

Test/fix parsing of psuedoclasses/elements.

Add support for psuedoelements/classes.

Code cleanliness fixes.

Improve code cleanliness to avoid future bugs.

Parse HTML 'style' attributes.