Let VS Code recognize decorators (#17258)

https://www.typescriptlang.org/tsconfig#experimentalDecorators

jsconfig.json

@@ -14,6 +14,7 @@
         "./enterprise/frontend/src/*",
         "./enterprise/frontend/test/*"
       ]
-    }
+    },
+    "experimentalDecorators": true
   }
 }