Add `features/layers` package

[Yota Hamada]

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