SyncShapesToTablesOptions onInitialSync type should allow async

[jay-babu]

export interface SyncShapesToTablesOptions {
  key: string | null
  shapes: Record<string, ShapeToTableOptions>
  useCopy?: boolean // DEPRECATED: use initialInsertMethod instead
  initialInsertMethod?: InitialInsertMethod
  onInitialSync?: () => void | Promise<void>
}

I think this would be better to show async support? It would also follow the example given in linear-lite

[jay-babu]

https://github.com/electric-sql/electric/blob/1c55d2acbda0b1e486511d6b27396b2363c2393a/examples/linearlite/src/sync.ts#L78