Add `EntityWorldMut::commands` method

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

I'm implementing UI widgets as `EntityCommand`s to get an API like this:

```rust
commands.spawn_empty().add(my_ui_widget).insert(Name::new("CustomName"))
```

I want my parent widget to create child widgets with `entity.add(my_child_widget)`, but `entity` is an `EntityWorldMut`, not an `EntityCommands`, so it has no `.add` method.

## What solution would you like?

```rust
fn my_parent_widget(mut entity: EntityWorldMut) {
entity.commands().add(my_child_widget).insert(...);
}
```

This is a natural solution, because `EntityWorldMut::commands -> EntityCommands` would parallel `World::commands -> Commands`.

## What alternative(s) have you considered?

1. `EntityWorldMut::add`:

```rust
fn my_parent_widget(mut entity: EntityWorldMut) {
entity.add(my_child_widget).insert(...);
}
```

2. Just work around it:

```rust
fn my_parent_widget(mut entity: EntityWorldMut) {
{
let id = entity.id();
entity.world_scope(|world| world.commands().add(my_child_widget.with_entity(id)));
}
entity.insert(...);
}
```