✨ classic: adds relation initStore

[pietrovismara]

• Drops references to Schema as the store now can be anything
• Sets the initStore callback as $schema, so that Pair can use it
• Adds a test in Relation.test.ts

Unrelated: improves the prefab check in getRelationTargets

[NateTheGreatt]

great work thanks for this! couple change requests since relations don't need schemas anymore

[krispya]

Looks good to me. I'm merging.