Support for postcss-modules-values

Hi there! Is there a way in which I can make atom-pigments work with [postcss-modules-values](https://github.com/css-modules/postcss-modules-values)? I would love if it were possible somehow that in the same way that it recognizes all my less @variables with colors across files, it would do that to these.

It works like this:
```
/* colors.css */
@value primary: #BF4040;
@value secondary: #1F4F7F;

.text-primary {
color: primary;
}

.text-secondary {
color: secondary;
}
```

As you can see, there is no @ or $ before calling up the value. This is the same when you call up variables cross-file:

```
/* header.css */
@value colors: "./colors.css";
@value primary, secondary from colors;

.header {
composes: primary from colors;
box-shadow: 0 0 10px secondary;
}
```

I would like the `primary` and `secondary` to have the color they were given in `colors.css`, also when they're imported into a different file.

Let me know if I can do anything to make this happen!

The issue is about extending atom-pigments to recognize postcss-modules-values syntax like `@value primary: #BF4040;` and `color: primary;`. Start by examining how the package currently parses Less variables and other color definitions. Look for parsing logic in the source files, likely in lib/ or spec/. Test the change by creating a sample project with postcss-modules-values and checking if colors are highlighted. The goal is to have the plugin highlight colors defined with @value across CSS files.

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