Add additional root placeholders in a code-first JSS sample app

By default, JSS applications define only one root placeholder in the Layout component, but you can add multiple root placeholders if your use case requires it.

To define multiple root placeholders:

1. Add additional placeholder instances in your JSS Layout component, giving each placeholder a unique name.
2. Add the placeholder settings items to the manifest.
3. In the package.json file, in the config options, list the placeholders in the rootPlaceholders array.