`<xyz-params>` fails to include `<percentage>`

[svgeesus]

(Originally reported by @mirisuzanne in Twitter DM):

The syntax for the color() function allows a mix of <number>, <percentage> and none for the RGB spaces

<predefined-rgb-params> = <predefined-rgb> [ <number> | <percentage> | none ]{3}

but has not been updated for the XYZ spaces

<xyz-params> = <xyz-space> [ <number> | none ]{3}

Despite that, 10.8. The Predefined CIE XYZ Color Spaces lists reference ranges for percents.

So, the grammar should be updated to match the prose.

Mixed numbers and percents in the XYZ spaces are already tested on WPT by css-color/parsing/color-mixed-num-pct.html