Register type BoxShadow (#16750)

# Objective

- Register `BoxShadow` type for reflection

## Testing

- Tested that box shadow example compiles and runs

## Additional

- It would be nice to have this in 0.15.1

crates/bevy_ui/src/lib.rs

@@ -170,6 +170,7 @@ impl Plugin for UiPlugin {
             .register_type::<UiScale>()
             .register_type::<BorderColor>()
             .register_type::<BorderRadius>()
+            .register_type::<BoxShadow>()
             .register_type::<widget::Button>()
             .register_type::<widget::Label>()
             .register_type::<ZIndex>()