bevy_scene: Add missing registration for SmallVec<[Entity; 8]> (#6578)

# Objective

> Part of #6573

`Children` was not being properly deserialized in scenes. This was due to a missing registration on `SmallVec<[Entity; 8]>`, which is used by `Children`.

## Solution

Register `SmallVec<[Entity; 8]>`.

---

## Changelog

- Registered `SmallVec<[Entity; 8]>`

crates/bevy_hierarchy/src/lib.rs

@@ -41,6 +41,7 @@ impl Plugin for HierarchyPlugin {
     fn build(&self, app: &mut App) {
         app.register_type::<Children>()
             .register_type::<Parent>()
+            .register_type::<smallvec::SmallVec<[bevy_ecs::entity::Entity; 8]>>()
             .add_event::<HierarchyEvent>();
     }
 }