feat: Implement contrast-color function

[LinusU]

Hello! 👋

This is my first time contributing, so I'm sure that I got a lot of things wrong. But I'm very happy to receive feedback and iterate on this!

This adds support for the contrast-color function. I've loosely based my changes on #105 which adds color-mix (huge thank you @devongovett 🙏).

I've followed the current draft specification: https://drafts.csswg.org/css-color-6/#colorcontrast