Feature: Prevent a node from being exported to JSON

I'd like to be able to prevent a custom node from being exported to JSON, similar to how a custom node can be prevented from exporting to HTML ([by setting element to null](https://lexical.dev/docs/concepts/serialization#lexical---html))

I've tried implementing my own custom serializer, but this doesn't feel good and will likely break as lexical continues to be developed.

Let me know if I am misunderstanding a better way to do this, thanks!