Implement insert_children for EntityCommands (#18675)

Extension of #18409.

I was updating a migration guide for hierarchy commands and realized
`insert_children` wasn't added to `EntityCommands`, only
`EntityWorldMut`.

This adds that and `insert_related` (basically just some
copy-and-pasting).

crates/bevy_ecs/src/hierarchy.rs

@@ -361,6 +361,12 @@ impl<'a> EntityCommands<'a> {
         self.add_related::<ChildOf>(children)
     }
 
+    /// Insert children at specific index.
+    /// See also [`insert_related`](Self::insert_related).
+    pub fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
+        self.insert_related::<ChildOf>(index, children)
+    }
+
     /// Adds the given child to this entity
     pub fn add_child(&mut self, child: Entity) -> &mut Self {
         self.add_related::<ChildOf>(&[child])

crates/bevy_ecs/src/relationship/related_methods.rs

@@ -343,6 +343,23 @@ impl<'a> EntityCommands<'a> {
         })
     }
 
+    /// Relates the given entities to this entity with the relation `R`, starting at this particular index.
+    ///
+    /// If the `related` has duplicates, a related entity will take the index of its last occurrence in `related`.
+    /// If the indices go out of bounds, they will be clamped into bounds.
+    /// This will not re-order existing related entities unless they are in `related`.
+    pub fn insert_related<R: Relationship>(&mut self, index: usize, related: &[Entity]) -> &mut Self
+    where
+        <R::RelationshipTarget as RelationshipTarget>::Collection:
+            OrderedRelationshipSourceCollection,
+    {
+        let related: Box<[Entity]> = related.into();
+
+        self.queue(move |mut entity: EntityWorldMut| {
+            entity.insert_related::<R>(index, &related);
+        })
+    }
+
     /// Relates the given entity to this with the relation `R`.
     ///
     /// See [`add_related`](Self::add_related) if you want to relate more than one entity.