Update the Children component of the parent entity when a scene gets deleted

[cBournhonesque]

Objective

• A scene usually gets created using the SceneBundle or DynamicSceneBundle. This means that the scene's entities get added as children of the root entity (the entity on which the SceneBundle gets added)
• When the scene gets deleted using the SceneSpawner, the scene's entities are deleted, but the Children component of the root entity doesn't get updated. This means that the hierarchy becomes unsound, with Children linking to non-existing components.

Solution

• Update the despawn_sync logic to also update the Children from any parents of the scene, if there are any
• Adds a test where a Scene gets despawned and checks for dangling Children references on the parent. The test fails on main but works here.

Alternative implementations

• One option could be to add a parent: Option<Entity> on the InstanceInfo struct that tracks if the SceneInstance was added as a child of a root entity

[alice-i-cecile]

#12235 may obsolete this PR.

[james7132]

Thanks for adding an test for this!