Soft child insertion (for example, try_add_child)

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

```
Encountered an error in command `::add_related:
:{{closure}} as bevy_ecs::error::command_handling::CommandWithEntity>>::with_entity::{{closure}}`: The entity with ID 986v5 does not exist (enable `track_location` feature for more details)
```

Which crashes the app.

## What solution would you like?

A function like `try_insert` compared to `insert` where it logs a warning instead of panicing.

## What alternative(s) have you considered?

I'm currently a custom command someone provided on the Discord server, but thought it might be benefitial for others if something like that existed on the official repo. Here's what I'm currently using:
```
commands.queue(move |world: &mut World| {
let Ok(mut ec) = world.get_entity_mut(sprite_entity) else {
return;
};
ec.add_child((
// child here
));
});
```

Start by reading the existing EntityCommands add_related and insert behavior, along with the custom World command shown in the issue. Define the soft-insertion behavior around missing entities, then verify that it warns instead of panicking while preserving normal child insertion.

Written by the indexing model from the issue text.