Fix with_child not inserting Parent component (#15009)

# Objective

The `Parent` component holds a reference to the parent entity of the
entity it is inserted onto. The `with_child` function erroneously
forgets to insert this component onto the child entity that it spawns,
causing buggy behaviour when the function is used instead of the other
child-spawning functions.

## Solution

Ensure `with_child` inserts the `Parent` component, the same as all the
other child-spawning functions.

## Testing

Checked before/after with a bevy_ui layout where this patch fixed buggy
behaviour I was seeing in parent/child UI nodes.

crates/bevy_hierarchy/src/child_builder.rs

@@ -585,7 +585,8 @@ impl BuildChildren for EntityWorldMut<'_> {
  }
 
  fn with_child<B: Bundle>(&mut self, bundle: B) -> &mut Self {
- let child = self.world_scope(|world| world.spawn(bundle).id());
+ let parent = self.id();
+ let child = self.world_scope(|world| world.spawn((bundle, Parent(parent))).id());
  if let Some(mut children_component) = self.get_mut::<Children>() {
  children_component.0.retain(|value| child != *value);
  children_component.0.push(child);