Deferred Entity Removal

[Yota Hamada]

Add a safe mechanism to remove entities because accessing a removed entity would cause panic.

• Create features/commands package
• Create a queuing mechanism for processing commands
• Create RemoveRecursive(world World, entry *donburi.Entry) function
• Create ProcessCommands(world World) function
• Update the ecs.ECS.Update() method to call ProcessCommands()