Support `color(srgb 0 0 0 / 0.5)` syntax

[unional]

This is one of the formats returned by Chrome (e.g. when using color-mix()).

Currently it fails with:

Error: unknown format: color(srgb 0 0 0 / 0.5)
    at new Color 

[Bilge]

+1. Firefox is returning this format for getComputedStyle(temp0).backgroundColor, e.g. "color(srgb 0.639216 0.419608 1 / 0.298)". If this is a browser-native format, it seems logical it should be supported, otherwise this becomes difficult to parse.