feat: add var() resolution and mode-* descriptors

[artmsilva]

Summary

• Resolve var(--X) references in initial-value by chasing through the token graph until a literal is found. Handles chains (A→B→C) and circular references (kept as-is).
• Add mode-* extended descriptors (e.g., mode-dark, mode-high-contrast) to express color mode overrides directly in @property blocks. Mode values also get var() resolution.

Authoring

@property --color-action-primary {
  syntax: "<color>";
  inherits: true;
  initial-value: var(--color-blue-450);
  mode-dark: var(--color-blue-300);
}

Output

JSON (cssmark build): resolved literals

{
  "name": "--color-action-primary",
  "initialValue": "#2C49EF",
  "modes": { "dark": "#94B4FF" }
}

CSS (cssmark css): preserves var() references + generates mode blocks

:root {
  --color-action-primary: var(--color-blue-450);
}

:root[data-color-mode='dark'] {
  --color-action-primary: var(--color-blue-300);
}