Add SpawnScene to prelude (#9451)

# Objective

#9260 added this schedule, but it's not in prelude like other schedules.

## Solution

Add to prelude.

crates/bevy_app/src/lib.rs

@@ -26,7 +26,7 @@ pub mod prelude {
         app::App,
         main_schedule::{
             First, FixedUpdate, Last, Main, PostStartup, PostUpdate, PreStartup, PreUpdate,
-            Startup, StateTransition, Update,
+            SpawnScene, Startup, StateTransition, Update,
         },
         DynamicPlugin, Plugin, PluginGroup,
     };