[css-color-5] color-function used but not defined

[svgeesus]

In Mixing colors: the color-mix function and also Adjusting colors: the color-adjust function, the term <color-function> is used, but is not defined.

The following functions are defined informally, by being used in the examples:

• lightness()
• hue()
• a()
• b()

I assume that this one will also exist

• chroma()

When another colorspace than lch() or lab() is used, there will also be color functions for those spaces (rgb(), hsl(), hwb()).

[svgeesus]

Dup of #4743