Despawn the player when exiting the Playing state

Closes #21: Despawn player ship when exiting `GameState::Playing`

I've also moved the `fn despawn<T>` utility to the lib.rs module.

src/lib.rs

@@ -51,6 +51,7 @@ impl Plugin for AsteroidPlugin {
         .init_resource::<GameAssets>()
         .add_systems(Startup, spawn_camera)
         .add_systems(OnEnter(GameState::Playing), objects::spawn_player)
+        .add_systems(OnExit(GameState::Playing), despawn::<Ship>)
         .add_systems(
             FixedUpdate,
             (
@@ -84,6 +85,14 @@ impl Plugin for AsteroidPlugin {
     }
 }
 
+/// Despawns entities matching the generic argument. Intended to remove UI
+/// elements.
+pub(crate) fn despawn<T: Component>(mut commands: Commands, to_despawn: Query<Entity, With<T>>) {
+    for entity in to_despawn {
+        commands.entity(entity).despawn();
+    }
+}
+
 /// The game's main state tracking mechanism, registered with Bevy as a [`State`](`bevy::prelude::State`).
 #[derive(Clone, Debug, Eq, Hash, PartialEq, States)]
 pub enum GameState {