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/loaders/sass.js

@@ -1,23 +0,0 @@
-const MiniCssExtractPlugin = require('mini-css-extract-plugin');
-
-module.exports = {
-  test: /\.s?css$/i,
-  use: [
-    MiniCssExtractPlugin.loader,
-    'css-loader',
-    {
-      loader: 'postcss-loader',
-      options: {
-        sourceMap: true,
-      },
-    },
-    {
-      loader: 'sass-loader',
-      options: {
-        fiber: require('fibers'),
-        implementation: require('sass'),
-        sourceMap: true,
-      },
-    },
-  ],
-};