Child "component" for required components

## What problem does this solve or what need does it fill?
A common part (as far as my code base is concerned, and looking at a few test projects I could find online) is creating entities and then adding children to said entity, to create a hierarchical system to a project. A useful feature would be to add a component that can be tagged onto a require set, allowing for a type to be automatically added to the entity when created as a child.

## What solution would you like?
```rust
#[derive(Component)]
#[require(Child)]
struct ExampleComponent;
```

## What alternative(s) have you considered?
The current solution to this project is just directly adding a child upon creating the entity at runtime, instead of assuring a compile time creation.