Introduce the ability to attach optional data payloads to
relationships. This includes:
- Updating `defineRelationship` to accept default values.
- Adding `getRelData` and `setRelData` to the `World` class.
- Allowing `relate` to accept an optional data override.
- Updating serialization to include relationship data in snapshots.
- Implementing lazy storage for relationship data using `SparseSet`.
Introduce `toJSON` and `fromJSON` methods to the `World` class to
allow saving and restoring world states. This requires components and
relationships to have human-readable names for stable serialization.
hypercross