Support the sass:selector module.

[kaj]

There should be complete support for sass:selector module. A little is done in #80, but much remains.

Fixing this may involve refactoring how css selectors are represented in the code, as most comparisons of selectors would probably benefit from some other representation than the current.

• is-superselector($super, $sub) (partially supported since v0.28.6).
• append($selectors...) (supported a long time, cleaned up in Use new selectors for selector.append. #190).
• extend($selector, $extendee, $extender) (mostly supported since Implement selector-extend function #194).
• nest($selectors...) (partial support since Support @use for builtin modules #80, full since Use logical selectors in nest. #189)
• parse($selector) (partially supported since v0.8.0, complete since Parse new selectors #192)
• replace($selector, $original, $replacement) (supported since Implement selector-replace function. #188)
• unify($selector1, $selector2) (partial support since Support the selector-unify function. #187)
• simple-selectors($selector) (Implement simple-selectors function. #191)