Feature Request: Conditions

[figsoda]

https://github.com/DeterminateSystems/riff/blob/159e7c5b7ee3770828cc0c936d939de5d1b58f83/registry/registry.json#L81-L94

This pattern can be seen quite a few times in the registry, it would be nice if this can be simplified to this, which is also more similar to how things are packaged in nixpkgs

{
  "core-founddation-sys": {
    "conditions": {
      "stdenv.isDarwin": {
        "build-inputs": [
          "darwin.apple_sdk.frameworks.CoreFoundation"
        ]
      }
    }
  }
}

I'm happy to try to implement this if someone gives me a green light