feat: add com.projectwallace.usage-count extension

[bartveneman]

closes #14

{
  colors: {
    "black-10a32d": {
      $value: {
        // etc.
      },
      $extensions: {
        "com.projectwallace.css-authored-as": "black",
        "com.projectwallace.usage-count": 14, // <-- this right here
      }
    }
  }
}