Replaced EntityCommand Implementation for FnOnce (#9604)

# Objective

- Fixes #4917
- Replaces #9602

## Solution

- Replaced `EntityCommand` implementation for `FnOnce` to apply to
`FnOnce(EntityMut)` instead of `FnOnce(Entity, &mut World)`

---

## Changelog

- `FnOnce(Entity, &mut World)` no longer implements `EntityCommand`.
This is a breaking change.

## Migration Guide

### 1. New-Type `FnOnce`

Create an `EntityCommand` type which implements the method you
previously wrote:

```rust
pub struct ClassicEntityCommand<F>(pub F);

impl<F> EntityCommand for ClassicEntityCommand<F>
where
    F: FnOnce(Entity, &mut World) + Send + 'static,
{
    fn apply(self, id: Entity, world: &mut World) {
        (self.0)(id, world);
    }
}

commands.add(ClassicEntityCommand(|id: Entity, world: &mut World| {
    /* ... */
}));
```

### 2. Extract `(Entity, &mut World)` from `EntityMut`

The method `into_world_mut` can be used to gain access to the `World`
from an `EntityMut`.

```rust
let old = |id: Entity, world: &mut World| {
    /* ... */
};

let new = |mut entity: EntityMut| {
    let id = entity.id();
    let world = entity.into_world_mut();
    /* ... */
};
```

crates/bevy_ecs/src/system/commands/mod.rs

@@ -5,7 +5,7 @@ use crate::{
     self as bevy_ecs,
     bundle::Bundle,
     entity::{Entities, Entity},
-    world::{FromWorld, World},
+    world::{EntityMut, FromWorld, World},
 };
 use bevy_ecs_macros::SystemParam;
 use bevy_utils::tracing::{error, info};
@@ -805,12 +805,13 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     ///
     /// ```
     /// # use bevy_ecs::prelude::*;
+    /// # use bevy_ecs::world::EntityMut;
     /// # fn my_system(mut commands: Commands) {
     /// commands
     ///     .spawn_empty()
     ///     // Closures with this signature implement `EntityCommand`.
-    ///     .add(|id: Entity, world: &mut World| {
-    ///         println!("Executed an EntityCommand for {id:?}");
+    ///     .add(|entity: EntityMut| {
+    ///         println!("Executed an EntityCommand for {:?}", entity.id());
     ///     });
     /// # }
     /// # bevy_ecs::system::assert_is_system(my_system);
@@ -848,10 +849,10 @@ where
 
 impl<F> EntityCommand for F
 where
-    F: FnOnce(Entity, &mut World) + Send + 'static,
+    F: FnOnce(EntityMut) + Send + 'static,
 {
     fn apply(self, id: Entity, world: &mut World) {
-        self(id, world);
+        self(world.entity_mut(id));
     }
 }