Allow entities to be added to a relationship using the new spawn api

[Freyja-moth]

Objective

Fixes #18662

Useful if you want to to add something that already exists to a relationship .

Solution

Added SpawnEntity and SpawnEntites which implement SpawnableList.

I also added SpawnIter and SpawnWith to bevy_ecs's prelude as it felt weird that they'd be missing, but maybe there's some reason why they weren't added? If you know anything about it lmk.

I'm also not entirely sold on the names so if anyone has any better ideas lmk.

Testing

Tested by adding a child using the new methods and checking if it exists, and it worked! First try!

Showcase

let child1 = commands.spawn(Name::new("Child1")).id();

commands.spawn((
    Name::new("Root"),
    Children::spawn((
        SpawnEntity(child1),
    )),
));

[grind086]

Maybe SpawnedEntity/SpawnedEntities to indicate that they're already spawned. Or something like WithRelated.

[BenjaminBrienen]

Update PR description code example with new terminology