feat(ecs): added a simple prefab system

crates/comet_ecs/src/prefabs.rs

@@ -0,0 +1,46 @@
+use comet_structs::FlatMap;
+
+pub type PrefabFactory = fn(&mut crate::Scene) -> usize;
+
+pub(crate) struct PrefabManager {
+    pub(crate) prefabs: FlatMap<String, PrefabFactory>,
+}
+
+impl PrefabManager {
+    pub fn new() -> Self {
+        Self {
+            prefabs: FlatMap::new(),
+        }
+    }
+
+    pub fn register(&mut self, name: &str, factory: PrefabFactory) {
+        self.prefabs.insert(name.to_string(), factory);
+    }
+
+    pub fn has_prefab(&self, name: &str) -> bool {
+        self.prefabs.contains(&name.to_string())
+    }
+}
+
+#[macro_export]
+macro_rules! register_prefab {
+    ($scene:expr, $name:expr, $($component:expr),* $(,)?) => {
+        {
+            fn prefab_factory(scene: &mut $crate::Scene) -> usize {
+                let entity = scene.new_entity() as usize;
+                $(
+                    scene.add_component(entity, $component);
+                )*
+                entity
+            }
+            $scene.register_prefab($name, prefab_factory);
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! spawn_prefab {
+    ($scene:expr, $name:expr) => {
+        $scene.spawn_prefab($name)
+    };
+}