[color-swatch] Show deltas from other color

[LeaVerou]

Depends on #81

This is very useful primarily for <color-scale>, but could be useful in its own right.

It would allow passing another color via an attribute (name TBB, ideas: rel, compareto, vs) and it would show deltas from that color in its coordinates. It would also change the behavior of coords and would allow passing deltaE algorithms too.

Something like this: