How to properly write Laravel mix + webpack settings?

Question

webpack

there is a webpack.mix.js file

const mix = require('laravel-mix');
mix
  .js('resources/js/app.js', 'assets/js')
  .sass('resources/sass/app.scss', 'assets/css')
  .sass('resources/sass/media.scss', 'assets/css')
  .webpackConfig({
    postcss: {
      plugins: {
        cssnano: {
          preset: ['advanced', {
            autoprefixer: false,
            cssDeclarationSorter: false,
            zindex: false,
            discardComments: {
              removeAll: true
            }
          }]
        }
      },
      preset: {
        browsers: 'cover 99.5%',
        autoprefixer: true
      },

      order: 'cssnanoLast'
    },
  })
  .browserSync('site.test:8000');

how to correctly write the settings for webpackConfig so that it does not give such an error

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration has an unknown property 'postcss'. These properties are valid:
   object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry?, externals?, infrastructureLogging?, loader?, mode?, module?, name?, node?, optimization?, output?, parallelism?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, serve?, stats?, target?, watch?, watchOptions? }
   For typos: please correct them.
   For loader options: webpack >= v2.0.0 no longer allows custom properties in configuration.
     Loaders should be updated to allow passing options via loader options in module.rules.
     Until loaders are updated one can use the LoaderOptionsPlugin to pass these options to the loader:
     plugins: [
       new webpack.LoaderOptionsPlugin({
         // test: /\.xxx$/, // may apply this only for some modules
         options: {
           postcss: …
         }
       })
     ]