Add and use a custom canvas element on the page

Tell bevy to find the canvas with ID "#boids-canvas" instead of creating
it's own.

src/main.rs

@@ -10,6 +10,7 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins.set(WindowPlugin {
             primary_window: Some(Window {
+                canvas: Some("#boids-canvas".to_owned()),
                 fit_canvas_to_parent: true,
                 prevent_default_event_handling: false,
                 ..default()