Implement basic gun controller

The spacebar fires the gun, bullets are spawned on top of the ship with
a velocity that sends them forward (relative to the ship).

I still need a despawn mechanism, and a fire rate control. To despawn
things, I'm already planning a `Lifetime` component. For rate of fire,
an additional component will be created and attached to the player ship.

src/ship.rs

@@ -9,6 +9,9 @@ use bevy_rapier2d::prelude::*;
 #[derive(Component)]
 pub struct Ship;
 
+#[derive(Component)]
+pub struct Bullet;
+
 pub fn spawn_player(mut commands: Commands, game_assets: Res<GameAssets>) {
     commands
         .spawn((