Use spawn_batch in many_lights example (bevyengine#11979)

# Objective

- The `many_lights` example uses a for-loop around `commands.spawn`.
- It is generally recommended to use `spawn_batch` instead to lazily
spawn entities, because it doesn't massively grow the command queue.

## Solution

- Use `spawn_batch` in `many_lights` example.

---

## Discussion

- `thread_rng` is called for each light spawned. This is a simple
thread-local `Rc` clone, so it should compile down to a copy and an
increment + decrement instruction.
- I created `golden_ration` outside of the closure and `move`d it in.
This should just be a copy and hopefully will get const-evaluated away.
Would it be better to just move it into the closure itself?

## Performance

Using `spawn_batch` seems to decrease time-to-first-`Update` by 0.1s:
1.3s to 1.2s.

<details>
  <summary>Raw data and how it was collected.</summary>

Before:

- 2024-02-19T15:18:57.650987Z to 2024-02-19T15:18:58.912244Z : 1.3
- 2024-02-19T15:19:25.277135Z to 2024-02-19T15:19:26.542092Z : 1.3
- 2024-02-19T15:19:46.841460Z to 2024-02-19T15:19:48.137560Z : 1.3

After:

- 2024-02-19T15:17:05.749521Z to 2024-02-19T15:17:06.993221Z : 1.2
- 2024-02-19T15:17:38.153049Z to 2024-02-19T15:17:39.393760Z : 1.2
- 2024-02-19T15:18:10.691562Z to 2024-02-19T15:18:11.891430Z : 1.2

To time performance, I tracked the time from the first `Startup` logged
message to the first `Update` logged message.

```shell
$ cargo run --release --example many_lights
Compiling bevy v0.13.0 (/Users/bdeep/dev/bevy/bevy)
    Finished release [optimized] target(s) in 1.54s
     Running `target/release/examples/many_lights`
# THIS TIME
2024-02-19T15:30:13.429609Z  INFO bevy_render::renderer: AdapterInfo { name: "Apple M1", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
2024-02-19T15:30:13.566856Z  INFO bevy_winit::system: Creating new window "many_lights" (0v1)
2024-02-19T15:30:13.592371Z  WARN many_lights: This is a stress test used to push Bevy to its limit and debug performance issues. It is not representative of an actual game. It must be run in release mode using --release or it will be very slow.
2024-02-19T15:30:13.592572Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 14.2.1 ", kernel: "23.2.0", cpu: "Apple M1", core_count: "8", memory: "16.0 GiB" }
# TO THIS TIME
2024-02-19T15:30:15.429900Z  INFO many_lights: Lights: 100000
2024-02-19T15:30:15.430139Z  INFO bevy diagnostic: fps        :    0.982693   (avg 43.026557)
2024-02-19T15:30:15.430157Z  INFO bevy diagnostic: frame_time : 1017.611750ms (avg 149.456476ms)
2024-02-19T15:30:15.430165Z  INFO bevy diagnostic: frame_count:   12.000000   (avg 6.000000)
```

</details>

examples/stress_tests/many_lights.rs

@@ -69,11 +69,15 @@ fn setup(
     // the same number of visible meshes regardless of the viewing angle.
     // NOTE: f64 is used to avoid precision issues that produce visual artifacts in the distribution
     let golden_ratio = 0.5f64 * (1.0f64 + 5.0f64.sqrt());
-    let mut rng = thread_rng();
-    for i in 0..N_LIGHTS {
+
+    // Spawn N_LIGHTS many lights
+    commands.spawn_batch((0..N_LIGHTS).map(move |i| {
+        let mut rng = thread_rng();
+
         let spherical_polar_theta_phi = fibonacci_spiral_on_sphere(golden_ratio, i, N_LIGHTS);
         let unit_sphere_p = spherical_polar_to_cartesian(spherical_polar_theta_phi);
-        commands.spawn(PointLightBundle {
+
+        PointLightBundle {
             point_light: PointLight {
                 range: LIGHT_RADIUS,
                 intensity: LIGHT_INTENSITY,
@@ -82,8 +86,8 @@ fn setup(
             },
             transform: Transform::from_translation((RADIUS as f64 * unit_sphere_p).as_vec3()),
             ..default()
-        });
-    }
+        }
+    }));
 
     // camera
     match std::env::args().nth(1).as_deref() {