impl Command for <impl FnOnce(&mut World)> (#2996)

This is my first contribution to this exciting project! Thanks so much for your wonderful work. If there is anything that I can improve about this PR, please let me know :)

# Objective

- Fixes #2899
- If a simple one-off command is needed to be added within a System, this simplifies that process so that we can simply do `commands.add(|world: &mut World| { /* code here */ })` instead of defining a custom type implementing `Command`.

## Solution

- This is achieved by `impl Command for F where F: FnOnce(&mut World) + Send + Sync + 'static` as just calling the function.

I am not sure if the bounds can be further relaxed but needed the whole `Send`, `Sync`, and `'static` to get it to compile.

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

@@ -544,6 +544,15 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     }
 }
 
+impl<F> Command for F
+where
+    F: FnOnce(&mut World) + Send + Sync + 'static,
+{
+    fn write(self, world: &mut World) {
+        self(world)
+    }
+}
+
 #[derive(Debug)]
 pub struct Spawn<T> {
     pub bundle: T,
@@ -760,6 +769,10 @@ mod tests {
     #[derive(Component)]
     struct W<T>(T);
 
+    fn simple_command(world: &mut World) {
+        world.spawn().insert_bundle((W(0u32), W(42u64)));
+    }
+
     #[test]
     fn commands() {
         let mut world = World::default();
@@ -788,6 +801,27 @@ mod tests {
             .map(|(a, b)| (a.0, b.0))
             .collect::<Vec<_>>();
         assert_eq!(results2, vec![]);
+
+        // test adding simple (FnOnce) commands
+        {
+            let mut commands = Commands::new(&mut command_queue, &world);
+
+            // set up a simple command using a closure that adds one additional entity
+            commands.add(|world: &mut World| {
+                world.spawn().insert_bundle((W(42u32), W(0u64)));
+            });
+
+            // set up a simple command using a function that adds one additional entity
+            commands.add(simple_command);
+        }
+        command_queue.apply(&mut world);
+        let results3 = world
+            .query::<(&W<u32>, &W<u64>)>()
+            .iter(&world)
+            .map(|(a, b)| (a.0, b.0))
+            .collect::<Vec<_>>();
+
+        assert_eq!(results3, vec![(42u32, 0u64), (0u32, 42u64)]);
     }
 
     #[test]