Bump webpacker from 3.5.5 to 4.0.2 (#10277)

Bumps [webpacker](https://github.com/rails/webpacker) from 3.5.5 to 4.0.2.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v3.5.5...v4.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>
Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

config/webpack/rules/css.js

@@ -0,0 +1,30 @@
+const MiniCssExtractPlugin = require('mini-css-extract-plugin');
+
+module.exports = {
+  test: /\.s?css$/i,
+  use: [
+    MiniCssExtractPlugin.loader,
+    {
+      loader: 'css-loader',
+      options: {
+        sourceMap: true,
+        importLoaders: 2,
+        localIdentName: '[name]__[local]___[hash:base64:5]',
+      },
+    },
+    {
+      loader: 'postcss-loader',
+      options: {
+        sourceMap: true,
+      },
+    },
+    {
+      loader: 'sass-loader',
+      options: {
+        fiber: require('fibers'),
+        implementation: require('sass'),
+        sourceMap: true,
+      },
+    },
+  ],
+};