Bevy creates it's own canvas element by default, but the custom page
defines its own. This makes Bevy go find that one.
The window sizing still works -- it simply resizes the canvas upon
starting. I'm not sure if I'll change that or remove the size from the
HTML. That's a tomorrow problem.
The WorldSize is a Resource so that I can use it in the entity wrapping
system. The size is pulled from a public constant in the config.rs file,
and the window is made that same size.
The ship, such as it is, exists and moves when the player presses "W".
Position is updated according to velocity, and the mesh transform is
updated to match the position.
