webpack + autoprefixerでベンダープレフィックス付与させたい。

Question

package.json

{
  "name": "starter-theme",
  "version": "1.0.0",
  "description": "",
  "private": true,
  "scripts": {
    "build": "webpack --mode=production",
    "build:dev": "webpack --mode=development --watch"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/core": "^7.7.7",
    "@babel/preset-env": "^7.7.7",
    "autoprefixer": "^9.8.4",
    "babel-loader": "^8.0.6",
    "css-loader": "^3.4.0",
    "jquery": "^3.4.1",
    "mini-css-extract-plugin": "^0.9.0",
    "node-sass": "^4.13.0",
    "optimize-css-assets-webpack-plugin": "^5.0.3",
    "postcss-loader": "^3.0.0",
    "sass-loader": "^8.0.0",
    "style-loader": "^1.1.2",
    "terser-webpack-plugin": "^2.3.1",
    "webpack": "^4.41.5",
    "webpack-cli": "^3.3.10"
  }
}

webpack.config.js

const path = require("path");
const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const TerserPlugin = require("terser-webpack-plugin");
const OptimizeCssAssetsPlugin = require("optimize-css-assets-webpack-plugin");

module.exports = (env, argv) => ({
  watch: true,
  entry: "./assets/js/src/index.js",
  output: {
    filename: "bundle.js",
    path: path.resolve(__dirname, "assets/js"),
  },

  // 最適化オプションを上書き
  optimization: {
    minimizer: [new TerserPlugin({}), new OptimizeCssAssetsPlugin({})],
  },

  // ソースマップの設定
  devtool: "source-map",

  module: {
    rules: [
      // babel-loaderの設定
      {
        test: /.js$/,
        use: [
          {
            loader: "babel-loader",
            options: {
              presets: ["@babel/preset-env"],
            },
          },
        ],
        exclude: /node_modules/,
      },
      // css/sass-loaderの設定
      {
        test: /.(sa|sc|c)ss$/,
        use: [
          MiniCssExtractPlugin.loader,
          {
            loader: "css-loader",
            options: {
              url: false,
              sourceMap: true,
            },
          },
          "sass-loader",
        ],
      },
      // PostCSSのための設定
      {
        loader: "postcss-loader",
        options: {
          // PostCSS側でもソースマップを有効にする
          sourceMap: true,
          plugins: [
            // Autoprefixerを有効化
            // ベンダープレフィックスを自動付与する
            require("autoprefixer")({
              grid: true
            })
          ]
        }
      }
    ],
  },

  plugins: [
    new MiniCssExtractPlugin({
      filename: "../styles/bundle.css",
    }),
  ],
});

npm run build:dev                                                        

> starter-theme@1.0.0 build:dev /Users/akihirohanai/My_project/3_ugr.hokkaido.jp/app/wp-content/themes/ver.3_theme
> webpack --mode=development --watch


webpack is watching the files…

Hash: 983db68a01c30db0ca32
Version: webpack 4.41.5
Time: 152ms
Built at: 2020/07/07 15:33:30
        Asset      Size  Chunks                   Chunk Names
    bundle.js  4.38 KiB    main  [emitted]        main
bundle.js.map  3.53 KiB    main  [emitted] [dev]  main
Entrypoint main = bundle.js bundle.js.map
[./assets/js/src/index.js] 591 bytes {main} [built] [failed] [1 error]

ERROR in ./assets/js/src/index.js
Module build failed (from ./node_modules/postcss-loader/src/index.js):
SyntaxError

(1:1) Unknown word

> 1 | import "../../styles/src/style.scss";
    | ^
  2 | import $ from "jquery";
  3 | import { slickslide } from "./modules/slick";

ちなみに、index.jsは以下の通り

import "../../styles/src/style.scss";
import $ from "jquery";
import { slickslide } from "./modules/slick";

slickslide();

webpack.config.jsの下記の記述を消すとエラーは起きません。

webpack.config.js

// PostCSSのための設定
     {
        loader: "postcss-loader",
        options: {
          // PostCSS側でもソースマップを有効にする
          sourceMap: true,
          plugins: [
            // Autoprefixerを有効化
            // ベンダープレフィックスを自動付与する
            require("autoprefixer")({
              grid: true
            })
          ]
        }
      }

タイトルの通り、webpack + autoprefixerでベンダープレフィックス付与させたいのですが、webpack.config.jsの記述方法をご教示頂けませんでしょうか？