`color()` syntax: don't allow omitted components

[danburzo]

The css-color spec has been changed to disallow missing components in the color() syntax.

This used to be correct per spec, long ago when color() also did custom color spaces and it was reasonable to omit some components and have them auto-filled with zero.

Now custom color spaces are in CSS Color 5, and clearly distinguished with a dashed-ident; while predefined color spaces are in CSS Color 4 and are wither an RGB space or an XYZ space. Both take 3 components, as the grammar makes clear.

There was some leftover prose about variable number of components, not up to date with the grammar change, which I just corrected.

See:

• https://bugs.webkit.org/show_bug.cgi?id=251152#c4
• https://wpt.fyi/results/css/css-color/parsing/color-invalid-color-function.html?label=master&label=experimental&aligned&view=subtest&q=css%20color%20invalid

[danburzo]

Fixed in #190