Add ship thruster sound

The player's Ship now has an AudioPlayer component constantly looping a
thruster sound effect. It starts paused and is only resumed when the
player fires the thruster.

As noted in the TODO comment at the top of the input_ship_thruster(...)
system, I need to figure out if I want to start using the `Single<>`
query parameter instead of a `Query<>` and then doing my own
null-ability checks (like what it does now).

src/objects.rs

@@ -168,6 +168,12 @@ pub fn spawn_player(mut commands: Commands, game_assets: Res<GameAssets>) {
             Mesh2d(game_assets.ship().0),
             MeshMaterial2d(game_assets.ship().1),
             Transform::default().with_scale(Vec3::new(20.0, 20.0, 20.0)),
+            AudioPlayer::new(game_assets.ship_thruster_sound()),
+            PlaybackSettings {
+                mode: bevy::audio::PlaybackMode::Loop,
+                paused: true,
+                ..Default::default()
+            },
         ))
         .with_child((
             Mesh2d(game_assets.thruster_mesh()),