✨ classic: added definePrefab and registerPrefab

[pietrovismara]

Unfortunately there are several formatting changes included as prettier took over a bunch of unformatted code.

The gist of the changes is the following:

• definePrefab returns a PrefabToken, containing a list of components and a Map<World, eid>
• registerPrefab assigns an eid to the prefab by internally calling addPrefab, and pairs the eid with the world in the PrefabToken world -> eid map
• Calling IsA (or any other relation) on a PrefabToken stores it in the relation's $pairsMap and the component's $pairTarget, as it would for an eid
• getRelationTargets checks if $pairTarget is a PrefabToken and converts it to an eid, using the world -> eid map of PrefabToken

I left the definition of the Prefab component in Entity.ts, as moving it out of there would cause circular dependency issues. I suggest a separate refactor to prevent such issues.

See the included integration test (Prefab.test.ts) for usage.