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.

ISSUES: Record pseudoclass progress.

ISSUES: Plan psuedoclass lowering.

ISSUES: Text properties *mostly* implemented, upstreamed from Rhapsode core.

Moved to personal git homeserver, copy ISSUES over from NZOSS.