Add insert_child and remove_child methods

[Joel-Singh]

Objective

• remove_child was mentioned missing in Add a despawn_children method #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 Add a despawn_children method to EntityWorldMut and EntityCommands #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