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/file.js

@@ -0,0 +1,20 @@
+const { join } = require('path');
+const { settings } = require('../configuration');
+
+module.exports = {
+  test: new RegExp(`(${settings.static_assets_extensions.join('|')})$`, 'i'),
+  use: [
+    {
+      loader: 'file-loader',
+      options: {
+        name(file) {
+          if (file.includes(settings.source_path)) {
+            return 'media/[path][name]-[hash].[ext]';
+          }
+          return 'media/[folder]/[name]-[hash:8].[ext]';
+        },
+        context: join(settings.source_path),
+      },
+    },
+  ],
+};