Missing a method to insert related entities at a specific index (insert_children)

[villor]

What problem does this solve or what need does it fill?

Before #17398 there was a pretty handy method for inserting a slice of children at a specific index: insert_children

Not sure if it was intentionally left out, or just lost in the process. It's pretty useful for UI sometimes when needing to insert one or more children in specific spots.

While migrating bevy_editor_prototypes I resorted to an extension for now:

impl InsertChildrenExt for EntityWorldMut<'_> {
    fn insert_children(&mut self, index: usize, children: &[Entity]) {
        let prev_children = self.take::<Children>().unwrap_or_default();
        let (prev_left, prev_right) = prev_children.split_at(index);

        let parent_id = self.id();
        self.world_scope(|world| {
            for child in prev_left.iter().chain(children).chain(prev_right) {
                world.entity_mut(*child).insert(ChildOf(parent_id));
            }
        });
    }
}

What solution would you like?

• Add insert_related method on EntityWorldMut/EntityCommands, inserting a &[Entity] (or iterator?) at a specific index.
• Add insert_children for convenience and less breakage when migrating

What alternative(s) have you considered?

• Could just skip it and let "user-space" handle it like the extension above or similar. But seeing as it was in there before, I guess at some point it was deemed useful enough to keep upstream.