Set webpack depedency in package.json

This fixes the following errors during the asset precompilation step

warning " > webpack-dev-server@3.11.0" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
warning "webpack-dev-server > webpack-dev-middleware@3.7.2" has unmet peer dependency "webpack@^4.0.0".

See rails/webpacker#1078

package.json

@@ -6,7 +6,8 @@
     "@rails/activestorage": "^6.0.0",
     "@rails/ujs": "^6.0.0",
     "@rails/webpacker": "4.2.2",
-    "turbolinks": "^5.2.0"
+    "turbolinks": "^5.2.0",
+    "webpack": "^4.0.0"
   },
   "version": "0.1.0",
   "devDependencies": {