feat: add world serialization support

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.

src/index.ts

@@ -14,3 +14,4 @@ export type {
   QueryUpdate,
   RelationshipUpdate,
 } from "./observable/events";
+export type { WorldSnapshot } from "./serialization";