Convert colors via Lab colorspace

[jonathantneal]

What makes gray() special is that it is perceptual. According to CSS Color Module Level 4:

gray() = gray( <number>  [, <alpha-value>]? )

The first argument specifies the shade of gray, equal to the CIE Lightness, while the second optional argument specifies the alpha channel of the gray.

And is specifically called out in the editor’s draft:

Note: In other words, gray(a / b) is equal to lab(a 0 0 / b)

This means we’ll want to use Lab to calculate the shade of gray. It looks like you are currently using RGB to determine the shade of gray.

While fixing this creates a breaking change, I do recommend changing this plugin to follow the specification.