Support Lab color

[justinfagnani]

The Lab color space is part of CSS color 4, and is implemented in Safari already. There are also some operations, like color distance, nicely defined in Lab space.

It's be great to support the lab() syntax in parsing, and toLab() and toLabString() methods.

[petr001]

Actually landing to all major browsers:
https://caniuse.com/css-lch-lab