Add basic enemy behavior and spawning system

The changes introduce enemy mechanics and management - including
spawning, movement patterns, shooting timers, and death handling.

scenes/main.gd

@@ -0,0 +1,20 @@
+extends Node2D
+
+
+var enemy = preload("res://scenes/enemy.tscn")
+var score = 0
+
+func _ready():
+    spawn_enemies()
+
+func spawn_enemies():
+    for x in range(9):
+        for y in range(3):
+            var e = enemy.instantiate()
+            var pos = Vector2(x * (16 + 8) + 24, 16 * 4 + y * 16)
+            add_child(e)
+            e.start(pos)
+            e.died.connect(_on_enemy_died)
+
+func _on_enemy_died(value):
+    score += value