Validate CSS variables name

[PavelFil]

Hello. This code doesn't show any error.

body {
    color: var(primary-color);
}

But property names must be prefixed with --.

[lahmatiy]

CSSTree currently skips validation of declaration values that contain var(...), but it would be beneficial to at least validate the variable names. It would be great if you could create an issue in CSSTree to address this improvement.