webpack.config.js

const webpack = require("webpack");
const defaultConfig = require("./node_modules/@wordpress/scripts/config/webpack.config");
const postcssPresetEnv = require("postcss-preset-env");
const path = require("path");

module.exports = {
  ...defaultConfig,
  entry: {
    ...defaultConfig.entry,
    frontend: path.resolve(process.cwd(), "src", "frontend.js")
  },
  module: {
    ...defaultConfig.module,
    rules: [
      ...defaultConfig.module.rules,
      {
        test: /\.(sc|sa|c)ss$/,
        use: [
          {
            loader: "file-loader",
            options: {
              name: "[name].css"
            }
          },
          {
            loader: "extract-loader"
          },
          {
            loader: "css-loader"
          },
          {
            loader: "sass-loader"
          },
          {
            loader: "postcss-loader",
            options: {
              ident: "postcss",
              plugins: () => [
                postcssPresetEnv({
                  stage: 3,
                  features: {
                    "custom-media-queries": {
                      preserve: false
                    },
                    "custom-properties": {
                      preserve: true
                    },
                    "nesting-rules": true
                  }
                })
              ]
            }
          }
        ]
      }
    ]
  }
};