Update state/states example to use children! macro (#18250)

# Objective

Contributes to #18238 
Updates the `state/states` example to use the `children!` macro. Note
that this example also requires `--features bevy_dev_tools`

## Solution

Updates examples to use the Improved Spawning API merged in
https://github.com/bevyengine/bevy/pull/17521

## Testing

- Did you test these changes? If so, how?
- Opened the examples before and after and verified the same behavior
was observed. I did this on Ubuntu 24.04.2 LTS using `--features
wayland`.
- Are there any parts that need more testing?
- Other OS's and features can't hurt, but this is such a small change it
shouldn't be a problem.
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
  - Run the examples yourself with and without these changes.
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
  - see above

---

## Showcase

n/a

## Migration Guide

n/a

examples/state/states.rs

@@ -51,17 +51,16 @@ fn setup(mut commands: Commands) {
 
 fn setup_menu(mut commands: Commands) {
     let button_entity = commands
-        .spawn(Node {
+        .spawn((
+            Node {
                 // center button
                 width: Val::Percent(100.),
                 height: Val::Percent(100.),
                 justify_content: JustifyContent::Center,
                 align_items: AlignItems::Center,
                 ..default()
-        })
+            },
-        .with_children(|parent| {
+            children![(
-            parent
-                .spawn((
                 Button,
                 Node {
                     width: Val::Px(150.),
@@ -73,18 +72,16 @@ fn setup_menu(mut commands: Commands) {
                     ..default()
                 },
                 BackgroundColor(NORMAL_BUTTON),
-                ))
+                children![(
-                .with_children(|parent| {
-                    parent.spawn((
                     Text::new("Play"),
                     TextFont {
                         font_size: 33.0,
                         ..default()
                     },
                     TextColor(Color::srgb(0.9, 0.9, 0.9)),
-                    ));
+                )],
-                });
+            )],
-        })
+        ))
         .id();
     commands.insert_resource(MenuData { button_entity });
 }