Add EntityWorldMut::reborrow_scope() (#18730)

# Objective

Allow `EntityCommand` implementors to delegate to other entity commands
easily:

```rs
impl EntityCommand for Foo {
    fn apply(self, mut entity: EntityWorldMut) {
        entity.reborrow_scope(|e| StepOne.apply(e));
        entity.reborrow_scope(|e| StepTwo.apply(e));
    }
}
```

crates/bevy_ecs/src/world/entity_ref.rs

@@ -2811,6 +2811,22 @@ impl<'w> EntityWorldMut<'w> {
             .entity_get_spawned_or_despawned_by(self.entity)
             .map(|location| location.unwrap())
     }
+
+    /// Reborrows this entity in a temporary scope.
+    /// This is useful for executing a function that requires a `EntityWorldMut`
+    /// but you do not want to move out the entity ownership.
+    pub fn reborrow_scope<U>(&mut self, f: impl FnOnce(EntityWorldMut) -> U) -> U {
+        let Self {
+            entity, location, ..
+        } = *self;
+        self.world_scope(move |world| {
+            f(EntityWorldMut {
+                world,
+                entity,
+                location,
+            })
+        })
+    }
 }
 
 /// A view into a single entity and component in a world, which may either be vacant or occupied.