Fix webpacker build

Author: mshibuya

spec/dummy_app/package.json

@@ -3,6 +3,8 @@
   "private": true,
   "version": "0.1.0",
   "dependencies": {
+    "@babel/plugin-proposal-private-methods": "^7.18.6",
+    "@babel/plugin-proposal-private-property-in-object": "^7.21.11",
     "@rails/actiontext": "^7.0.3-1",
     "@rails/activestorage": "^7.0.3-1",
     "@rails/webpacker": "5.4.3",