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.