Add `features/layers` package

[yohamta]

Decouple the layering functionality from the ecs package.

• [ ] Add features/layers package
• [ ] Add a function func LayerTag(world World, layer LayerID) donburi.IComponentType
• [ ] Add a function func Create(world World, components ...donburi.ComponentType) donburi.Entity
• [ ] Add a function func CreateMany(world World, components ...donburi.ComponentType)
• [ ] Add a function func AddTo(world World, layer LayerID, entry *donburi.Entry)
• [ ] Add a function func NewQuery(layer LayerID, filter Filter) *query.Query
• [ ] Update the ecs package to use the features/layer pckage