chore: frontend bump

[ajasnosz]

Low-risk dependency cleanup for frontend/packages/manager.

• Remove 5 unused dependencies: scriptjs, chai, enzyme, enzyme-adapter-react-16, react-test-renderer. None are imported anywhere in src/.
• Bump webpack-merge ^4.1.3 → ^6.0.1 and update both demo/webpack.standalone.config.js and demo/webpack.splunkapp.config.js for the v6 API change (v5 dropped the callable default export in favor of a
  named merge export).