extract option fully implemented

[hgl]

I have a fully implemented and tested cssnext fork in my extract branch that is able to this:

config.js

{
  "features": {
    "customProperties": {
      "variables": {
        "color": "#e00"
      }
    },
    "customMedia": {
      "extensions": {
        "mq2": "screen"
      }
    }
  }
}

style.css

:root {
    --width: calc(1px + 1px);
    --width2: var(--width);
}
@custom-media --mq (max-width: 30em);

cssnext --config config.js --extract style.css

=>

{
    "customProperties":{
        "width": "2px",
        "width2": "2px",
        "color": "#e00"
    },
    "customMedia":{
        "mq": "(max-width: 30em)",
        "mq2": "screen"
    }
}

From our previous discussion, you were worried the implementation will be a hack.

I just want to demonstrate it's not. I only added an append option to the custom properties and custom media plugin, which leaves js defined variable in the css source code.

Will you reconsider merging it? Comparing to defining variables in js, I think it's cleaner and easier to write.

[MoOx]

I think you have everything you want from the plugins currently used.

[hgl]

Yes, I will revise this PR soon. please consider merging csstools/postcss-custom-media#10

[MoOx]

Not sure it's relevant now. Feel free to reopen a PR if it is.