New DebugUI plugin

This plugin is meant to have some add-ons to interact with the
simulation to provide insight.

This first change adds a cursor under the mouse with the intention that
it's circle can be used as a reticule for targeting boids. Hovering over
a cluster and getting their CoM or velocities, for example.

src/main.rs

@@ -1,12 +1,15 @@
 use bevy::prelude::*;
 
 mod birdoids_plugin;
+mod debug_plugin;
 
 use birdoids_plugin::BoidsPlugin;
+use debug_plugin::BoidsDebugPlugin;
 
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
+        .add_plugins(BoidsDebugPlugin)
         .add_plugins(BoidsPlugin)
         .run();
 }