Bump webpacker from 4.0.7 to 4.2.0 (#12416)

* Bump webpacker from 4.0.7 to 4.2.0

Bumps [webpacker](https://github.com/rails/webpacker) from 4.0.7 to 4.2.0.
- [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/v4.0.7...v4.2.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

* Use NODE_ENV=tests instead of test, to work around async modules not having a chunk

config/webpack/tests.js

@@ -0,0 +1,8 @@
+// Note: You must restart bin/webpack-dev-server for changes to take effect
+
+const merge = require('webpack-merge');
+const sharedConfig = require('./shared.js');
+
+module.exports = merge(sharedConfig, {
+  mode: 'development',
+});