Add @babel/core dependency

Author: afshin

package.json

@@ -17,6 +17,7 @@
     "watch": "onchange 'notebook/static/**/!(*.min).js' 'notebook/static/**/*.less' 'bower.json' -- npm run build"
   },
   "devDependencies": {
+    "@babel/core": "^7.15.0",
     "@babel/preset-env": "^7.15.0",
     "@jupyterlab/apputils": "^3.1.3",
     "babel-loader": "^8.2.2",