Implement support for CSS light-dark() in css.defineVars

[efoken]

As suggested in the StyleX docs, modern browsers should use the light-dark() CSS function instead of theme-based overrides. This simplifies the code and removes the need to explicitly define dark mode themes.

This PR adds support for light-dark() in css.defineVars:

const tokens = css.defineVars({
  lightDarkColor: 'light-dark(blue, green)'
});

For React Native, this is functionally equivalent to defining separate values and resolving them via the @media (prefers-color-scheme: dark) Media Query.

In modern browsers, light-dark() also allows the color scheme to be controlled via the color-scheme CSS property, instead of relying on custom theme logic or duplicated Media Query-based definitions.

This aligns the implementation with the current StyleX recommendation and modern CSS color-scheme handling.

[cmrn]

Thanks for working on this - I'm keen to see light-dark() support land in RSD.

One issue with the current approach: on web, light-dark() resolves against the color-scheme set at that point in the tree, not the media query. Resolving from prefers-color-scheme alone won't match web behaviour - e.g. a subtree with color-scheme: dark inside a light page resolves to the dark value on web, but this implementation would resolve it to light.

To get parity we'd need to track the color-scheme setting within the tree, similar to how CSS custom properties or inherited styles are handled today.