BSN: scene.spawn() system ergonomics (#23868)

# Objective

Spawning a scene on startup is a common pattern. Lets make it easier to
do so!

## Solution

- Add `SpawnSystem` and `SpawnListSystem` traits that are implemented
for functions that return scenes / scene lists, and return a system that
spawns the scene / handles errors.

### Before
```rust
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(world: &mut World) -> Result {
    world.spawn_scene_list(bsn_list![Camera2d, ui()])?;
    Ok(())
}
```

### After
```rust
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, scene.spawn())
        .run();
}

fn scene() -> impl SceneList {
    bsn_list![Camera2d, ui()]
}
```

This cuts out some boilerplate. It also further encourages people to
define standalone "scene functions" rather than embedding them in code,
which is generally a good pattern.

crates/bevy_scene/src/lib.rs

@@ -509,7 +509,7 @@ pub mod prelude {
     pub use crate::{
         bsn, bsn_list, on, template_value, CommandsSceneExt, EntityCommandsSceneExt,
         EntityWorldMutSceneExt, PatchFromTemplate, PatchTemplate, Scene, SceneList,
-        ScenePatchInstance, WorldSceneExt,
+        ScenePatchInstance, SpawnListSystem, SpawnSystem, WorldSceneExt,
     };
 }
 
@@ -523,6 +523,7 @@ mod scene;
 mod scene_list;
 mod scene_patch;
 mod spawn;
+mod spawn_system;
 
 pub use bevy_scene_macros::*;
 pub use resolved_scene::*;
@@ -530,6 +531,7 @@ pub use scene::*;
 pub use scene_list::*;
 pub use scene_patch::*;
 pub use spawn::*;
+pub use spawn_system::*;
 
 use bevy_app::{App, Plugin, SceneSpawnerSystems, SpawnScene};
 use bevy_asset::AssetApp;

crates/bevy_scene/src/spawn_system.rs

@@ -0,0 +1,35 @@
+use crate::{Scene, SceneList, WorldSceneExt};
+use bevy_ecs::{error::Result, world::World};
+
+/// Returns a system that spawns the given [`Scene`]. This should generally only be added to
+/// schedules that run once, such as [`Startup`](bevy_app::Startup).
+pub trait SpawnSystem {
+    /// Returns a system that spawns the given [`Scene`]. This should generally only be added to
+    /// schedules that run once, such as [`Startup`](bevy_app::Startup).
+    fn spawn(self) -> impl FnMut(&mut World) -> Result;
+}
+
+impl<F: FnMut() -> S + Send + Sync + 'static, S: Scene> SpawnSystem for F {
+    fn spawn(mut self) -> impl FnMut(&mut World) -> Result {
+        move |world: &mut World| -> Result {
+            world.spawn_scene(self())?;
+            Ok(())
+        }
+    }
+}
+
+/// Returns a system that spawns the given [`SceneList`]. This should generally only be added to
+/// schedules that run once, such as [`Startup`](bevy_app::Startup).
+pub trait SpawnListSystem {
+    /// Returns a system that spawns the given [`SceneList`]. This should generally only be added to
+    /// schedules that run once, such as [`Startup`](bevy_app::Startup).
+    fn spawn(self) -> impl FnMut(&mut World) -> Result;
+}
+impl<F: FnMut() -> S + Send + Sync + 'static, S: SceneList> SpawnListSystem for F {
+    fn spawn(mut self) -> impl FnMut(&mut World) -> Result {
+        move |world: &mut World| -> Result {
+            world.spawn_scene_list(self())?;
+            Ok(())
+        }
+    }
+}

examples/scene/bsn.rs

@@ -4,13 +4,12 @@ use bevy::{prelude::*, text::FontSourceTemplate};
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
-        .add_systems(Startup, setup)
+        .add_systems(Startup, scene.spawn())
         .run();
 }
 
-fn setup(world: &mut World) -> Result {
-    world.spawn_scene_list(bsn_list![Camera2d, ui()])?;
-    Ok(())
+fn scene() -> impl SceneList {
+    bsn_list![Camera2d, ui()]
 }
 
 fn ui() -> impl Scene {

examples/ui/widgets/feathers_counter.rs

@@ -10,7 +10,6 @@ use bevy::{
         tokens, FeathersPlugins,
     },
     prelude::*,
-    scene::prelude::Scene,
     ui_widgets::Activate,
 };
 
@@ -31,7 +30,7 @@ fn main() {
         // Configure feathers to use the dark theme
         .insert_resource(UiTheme(create_dark_theme()))
         .insert_resource(Counter(0))
-        .add_systems(Startup, setup)
+        .add_systems(Startup, scene.spawn())
         .add_systems(
             Update,
             update_counter_text.run_if(resource_changed::<Counter>),
@@ -39,9 +38,8 @@ fn main() {
         .run();
 }
 
-fn setup(world: &mut World) -> Result {
-    world.spawn_scene_list(bsn_list![Camera2d, demo_root()])?;
-    Ok(())
+fn scene() -> impl SceneList {
+    bsn_list![Camera2d, demo_root()]
 }
 
 fn demo_root() -> impl Scene {

examples/ui/widgets/feathers_gallery.rs

@@ -26,7 +26,6 @@ use bevy::{
     },
     input_focus::{tab_navigation::TabGroup, AutoFocus, InputFocus},
     prelude::*,
-    scene::prelude::Scene,
     text::{EditableText, TextEdit, TextEditChange},
     ui::{Checked, InteractionDisabled},
     ui_widgets::{
@@ -64,14 +63,13 @@ fn main() {
             rgb_color: palettes::tailwind::EMERALD_800.with_alpha(0.7),
             hsl_color: palettes::tailwind::AMBER_800.into(),
         })
-        .add_systems(Startup, setup)
+        .add_systems(Startup, scene.spawn())
         .add_systems(Update, update_colors)
         .run();
 }
 
-fn setup(world: &mut World) -> Result {
-    world.spawn_scene_list(bsn_list![Camera2d, demo_root()])?;
-    Ok(())
+fn scene() -> impl SceneList {
+    bsn_list![Camera2d, demo_root()]
 }
 
 fn demo_root() -> impl Scene {

examples/ui/widgets/virtual_keyboard.rs

@@ -15,7 +15,7 @@ fn main() {
     App::new()
         .add_plugins((DefaultPlugins, FeathersPlugins))
         .insert_resource(UiTheme(create_dark_theme()))
-        .add_systems(Startup, setup)
+        .add_systems(Startup, scene.spawn())
         .run();
 }
 
@@ -23,9 +23,8 @@ fn on_virtual_key_pressed(virtual_key_pressed: On<VirtualKeyPressed<&'static str
     println!("key pressed: {}", virtual_key_pressed.key);
 }
 
-fn setup(world: &mut World) -> Result {
-    world.spawn_scene_list(bsn_list![Camera2d, keyboard()])?;
-    Ok(())
+fn scene() -> impl SceneList {
+    bsn_list![Camera2d, keyboard()]
 }
 
 fn keyboard() -> impl Scene {