Feature: Support storing Y.Map as a NodeState value

## Description

Currently, the NodeState/Yjs integration only allows "JSON" values to be stored as NodeState values. The NodeState itself is stores as a Y.Map, but you can not nest other shared data structures in it.

For situations where you would be collaborating on a subset of that value (e.g. a list of comments on a particular node) it would make sense to use a shared data type.

## Impact

It will be possible to do things with NodeState that you must currently do manually with Yjs outside of the lexical document.