npm install mini-css-extract-plugin@1.6.0 -D

const MiniCssExtractPlugin = require('mini-css-extract-plugin');
module.exports = {
//...
  module: {
    rules: [{
      test: /\.css$/,
      use: [MiniCssExtractPlugin.loader, 'css-loader']
    }]
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: 'style.css',
    })
  ]
};