wp-getting-started-with-webpack-4-15.js

...
const MiniCssExtractPlugin = require("mini-css-extract-plugin");

module.exports = {
    ...
    module: {
        rules: [
            ...
            {
                test: /\.css$/,
                use: [MiniCssExtractPlugin.loader, "css-loader"]
            }
        ]
    },
    plugins: [
        ...
        new MiniCssExtractPlugin({
            filename: "css/[name].[chunkhash].css"
        })
      ]
};