feat: implementation of contrast-color() from the CSS Color 5

[yisibl]

The former name of contrast-color() was color-contrast(). In CSS Color 5, its syntax has been simplified to:
contrast-color() = contrast-color( <color> )

Spec: https://drafts.csswg.org/css-color-5/#contrast-color

This function is part of interop-2026 and is already supported in Firefox 146、Safari 26 and Chrome 147.0.7707.0(experimental)

This implementation solely parses functions, enabling color values within contrast-color() to be compressed or simplified.

Part of #99

[yisibl]

@devongovett PTAL

[devongovett]

I think contrast-color with a known value should compute to either black or white at build time, similar to how we calculate the value of relative colors and color-mix. There was a previous PR that implemented this (#808), but it also supported parts of the level 6 spec which is not yet final. Perhaps we can scope that down a bit?

[yisibl]

This can be done in the next PR. For this PR, I want to focus on implementing the syntax-level parsing.

However, I believe the algorithm currently defined for contrast-color in L5 is subpar. It might not be worth implementing this algorithm; it's better to leave it to the browser to handle.

See https://webkit.org/blog/16929/contrast-color/#:~:text=you%E2%80%99ll%20find%20many%20of%20the%20pairings%20with%20mid%2Dtone%20background%20colors%20don%E2%80%99t%20result%20in%20enough%20contrast.

[devongovett]

This can be done in the next PR.

It would change what gets stored in the AST though right? We wouldn't need a CssColor::ContrastColor variant if we parsed it directly to a color.