Add insert_child and remove_child methods (#19622)

# Objective

- `remove_child` was mentioned missing in #19556 and I realized that
`insert_child` was also missing.
- Removes the need to wrap a single entity with `&[]` with
`remove_children` and `insert_children`
- Would have also added `despawn_children` but #19283 does so. 

## Solution

- Simple wrapper around `remove_related`

## Testing

- Added `insert_child` and `remove_child` tests analgous to
`insert_children` and `remove_children` and then ran `cargo run -p ci --
test`

crates/bevy_ecs/src/hierarchy.rs

@@ -294,6 +294,12 @@ impl<'w> EntityWorldMut<'w> {
         self.insert_related::<ChildOf>(index, children)
     }
 
+    /// Insert child at specific index.
+    /// See also [`insert_related`](Self::insert_related).
+    pub fn insert_child(&mut self, index: usize, child: Entity) -> &mut Self {
+        self.insert_related::<ChildOf>(index, &[child])
+    }
+
     /// Adds the given child to this entity
     /// See also [`add_related`](Self::add_related).
     pub fn add_child(&mut self, child: Entity) -> &mut Self {
@@ -305,6 +311,11 @@ impl<'w> EntityWorldMut<'w> {
         self.remove_related::<ChildOf>(children)
     }
 
+    /// Removes the relationship between this entity and the given entity.
+    pub fn remove_child(&mut self, child: Entity) -> &mut Self {
+        self.remove_related::<ChildOf>(&[child])
+    }
+
     /// Replaces all the related children with a new set of children.
     pub fn replace_children(&mut self, children: &[Entity]) -> &mut Self {
         self.replace_related::<ChildOf>(children)
@@ -374,6 +385,12 @@ impl<'a> EntityCommands<'a> {
         self.insert_related::<ChildOf>(index, children)
     }
 
+    /// Insert children at specific index.
+    /// See also [`insert_related`](Self::insert_related).
+    pub fn insert_child(&mut self, index: usize, child: Entity) -> &mut Self {
+        self.insert_related::<ChildOf>(index, &[child])
+    }
+
     /// Adds the given child to this entity
     pub fn add_child(&mut self, child: Entity) -> &mut Self {
         self.add_related::<ChildOf>(&[child])
@@ -384,6 +401,11 @@ impl<'a> EntityCommands<'a> {
         self.remove_related::<ChildOf>(children)
     }
 
+    /// Removes the relationship between this entity and the given entity.
+    pub fn remove_child(&mut self, child: Entity) -> &mut Self {
+        self.remove_related::<ChildOf>(&[child])
+    }
+
     /// Replaces the children on this entity with a new list of children.
     pub fn replace_children(&mut self, children: &[Entity]) -> &mut Self {
         self.replace_related::<ChildOf>(children)
@@ -641,6 +663,29 @@ mod tests {
         );
     }
 
+    #[test]
+    fn insert_child() {
+        let mut world = World::new();
+        let child1 = world.spawn_empty().id();
+        let child2 = world.spawn_empty().id();
+        let child3 = world.spawn_empty().id();
+
+        let mut entity_world_mut = world.spawn_empty();
+
+        let first_children = entity_world_mut.add_children(&[child1, child2]);
+
+        let root = first_children.insert_child(1, child3).id();
+
+        let hierarchy = get_hierarchy(&world, root);
+        assert_eq!(
+            hierarchy,
+            Node::new_with(
+                root,
+                vec![Node::new(child1), Node::new(child3), Node::new(child2)]
+            )
+        );
+    }
+
     // regression test for https://github.com/bevyengine/bevy/pull/19134
     #[test]
     fn insert_children_index_bound() {
@@ -698,6 +743,25 @@ mod tests {
         );
     }
 
+    #[test]
+    fn remove_child() {
+        let mut world = World::new();
+        let child1 = world.spawn_empty().id();
+        let child2 = world.spawn_empty().id();
+        let child3 = world.spawn_empty().id();
+
+        let mut root = world.spawn_empty();
+        root.add_children(&[child1, child2, child3]);
+        root.remove_child(child2);
+        let root = root.id();
+
+        let hierarchy = get_hierarchy(&world, root);
+        assert_eq!(
+            hierarchy,
+            Node::new_with(root, vec![Node::new(child1), Node::new(child3)])
+        );
+    }
+
     #[test]
     fn self_parenting_invalid() {
         let mut world = World::new();