CleanCSS and source maps

[glen-84]

It's now possible to use CleanCSS:

const CleanCss = require("clean-css");

new OptimizeCssAssetsPlugin({
    cssProcessor: CleanCss,
    cssProcessorOptions: {
        sourceMap: true
    }
})

... but source maps do not work.

CleanCSS.process is not returning the source map (map), and it's also not appending the sourceMappingURL comment to the processed CSS.

It seems like it also needs to pass the current/previous map when calling minify (see this comment for an example).