Implement SpawnableList for Vec<Bundle> (#18259)

# Objective

In updating examples to use the Improved Spawning API I got tripped up
on being able to spawn children with a Vec. I eventually figured out
that I could use `Children::spawn(SpawnIter(my_vec.into_iter()))` but
thought there might be a more ergonomic way to approach it. After
tinkering with it for a while I came up with the implementation here. It
allows authors to use `Children::spawn(my_vec)` as an equivalent
implementation.

## Solution

- Implements `<R: Relationship, B: Bundle SpawnableList<R> for Vec<B>`
- uses `alloc::vec::Vec` for compatibility with `no_std` (`std::Vec`
also inherits implementations against the `alloc::vec::Vec` because std
is a re-export of the alloc struct, thanks @bushrat011899 for the info
on this!)

## 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

- Optional: you may use the new API to spawn `Vec`s of `Bundle` instead
of using the `SpawnIter` approach.

Author: krunchington

crates/bevy_ecs/src/spawn.rs

@@ -2,11 +2,12 @@
 //! for the best entry points into these APIs and examples of how to use them.
 
 use crate::{
-    bundle::{Bundle, BundleEffect, DynamicBundle},
+    bundle::{Bundle, BundleEffect, DynamicBundle, NoBundleEffect},
     entity::Entity,
     relationship::{RelatedSpawner, Relationship, RelationshipTarget},
     world::{EntityWorldMut, World},
 };
+use alloc::vec::Vec;
 use core::marker::PhantomData;
 use variadics_please::all_tuples;
 
@@ -45,6 +46,17 @@ pub trait SpawnableList<R> {
     fn size_hint(&self) -> usize;
 }
 
+impl<R: Relationship, B: Bundle<Effect: NoBundleEffect>> SpawnableList<R> for Vec<B> {
+    fn spawn(self, world: &mut World, entity: Entity) {
+        let mapped_bundles = self.into_iter().map(|b| (R::from(entity), b));
+        world.spawn_batch(mapped_bundles);
+    }
+
+    fn size_hint(&self) -> usize {
+        self.len()
+    }
+}
+
 impl<R: Relationship, B: Bundle> SpawnableList<R> for Spawn<B> {
     fn spawn(self, world: &mut World, entity: Entity) {
         world.spawn((R::from(entity), self.0));

examples/tools/scene_viewer/morph_viewer_plugin.rs

@@ -273,21 +273,16 @@ fn detect_morphs(
         |(i, target): (usize, &Target)| target.text_span(AVAILABLE_KEYS[i].name, style.clone());
     spans.extend(detected.iter().enumerate().map(target_to_text));
     commands.insert_resource(WeightsControl { weights: detected });
-    commands
-        .spawn((
-            Text::default(),
-            Node {
-                position_type: PositionType::Absolute,
-                top: Val::Px(12.0),
-                left: Val::Px(12.0),
-                ..default()
-            },
-        ))
-        .with_children(|p| {
-            for span in spans {
-                p.spawn(span);
-            }
-        });
+    commands.spawn((
+        Text::default(),
+        Node {
+            position_type: PositionType::Absolute,
+            top: Val::Px(12.0),
+            left: Val::Px(12.0),
+            ..default()
+        },
+        Children::spawn(spans),
+    ));
 }
 
 pub struct MorphViewerPlugin;