Add zoom function

man it took me so long to find out that Rust has a `ref` keyword lol

src/game/runtime.rs

@@ -1,5 +1,8 @@
 use avian2d::prelude::*;
-use bevy::prelude::*;
+use bevy::{
+    input::mouse::{AccumulatedMouseScroll, MouseScrollUnit},
+    prelude::*,
+};
 
 use super::objects::Player;
 
@@ -38,3 +41,18 @@ pub fn move_camera(
 ) {
     camera.translation = camera.translation.lerp(player.translation, 0.05);
 }
+
+pub fn zoom_camera(
+    mut camera: Single<&mut Projection, With<IsDefaultUiCamera>>,
+    scroll: Res<AccumulatedMouseScroll>,
+) -> Result {
+    let Projection::Orthographic(ref mut projection) = **camera else {
+        return Err("Default camera was not orthographic".into());
+    };
+    let scroll_type_multiplier = match scroll.unit {
+        MouseScrollUnit::Line => 0.01,
+        MouseScrollUnit::Pixel => 0.001,
+    };
+    projection.scale = (projection.scale - scroll.delta.y * scroll_type_multiplier).clamp(0.1, 2.5);
+    Ok(())
+}