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/index.js

@@ -0,0 +1,14 @@
+const babel = require('./babel');
+const css = require('./css');
+const file = require('./file');
+const nodeModules = require('./node_modules');
+
+// Webpack loaders are processed in reverse order
+// https://webpack.js.org/concepts/loaders/#loader-features
+// Lastly, process static files using file loader
+module.exports = {
+  file,
+  css,
+  nodeModules,
+  babel,
+};