It's a simple one-shot sound clip that gets dispatched as one more part
of the ship impact routine.
The GameAssets struct has been updated to have an array of handles for
audio assets (just the one, for now).
This sound file, and the next several, are all from Kenney
(www.kenney.nl).
Closes#16
Bevy provides Deref and DerefMut derives, so that's nice. I'm not sure
it makes any sense to keep the field private since those derefs expose
the Vec2 anyway. I added an `impl Default` anyway, though.
