Unit tests & bug fixes for counter-renderers.

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

Parse @layer rules.

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

Test basics of counter substitution.

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

Evaluate attribute namespace selectors.

Parse namespace selectors.

Unittest conditional styles, add loadImports API to fit Rhapsode's use.

Test rules capturing of ConditionalStyleSheet.

Test CSS variables.

Fix syntax errors in ConditionalStyles

Rework psuedoelement infrastructure so they can be their own boxes.

Test/fix parsing of psuedoclasses/elements.

Tidyup testsuite code, according to newly enabled compiler warnings.

Fix parser freezes, blocking issue for Rhapsode.

Fix incorrect handling of invalid syntaxes.

Test override styles.

Test cascade logic.

Test/fix cascade logic for selector specificity.