Add basic enemy behavior and spawning system

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

scenes/enemy.gd

@@ -0,0 +1,41 @@
+extends Area2D
+
+signal died
+
+var start_pos = Vector2.ZERO
+var speed = 0
+
+@onready var screensize  = get_viewport_rect().size
+
+func start(pos):
+    speed = 0
+    position = Vector2(pos.x, -pos.y)
+    start_pos = pos
+    await get_tree().create_timer(randf_range(0.25, 0.55)).timeout
+    var tween = create_tween().set_trans(Tween.TRANS_BACK)
+    tween.tween_property(self, "position:y", start_pos.y, 1.4)
+    await tween.finished
+    $MoveTimer.wait_time = randf_range(5, 20)
+    $MoveTimer.start()
+    $ShootTimer.wait_time = randf_range(4, 20)
+    $ShootTimer.start()
+
+func _on_timer_timeout():
+    speed = randf_range(75, 100)
+
+func _on_shoot_timer_timeout():
+    $ShootTimer.wait_time = randf_range(4, 20)
+    $ShootTimer.start()
+
+func _process(delta):
+    position.y += speed * delta
+    if position.y > screensize.y + 32:
+      start(start_pos)
+
+func explode():
+    speed = 0
+    $AnimationPlayer.play("explode")
+    set_deferred("monitoring", false)
+    died.emit(5)
+    await $AnimationPlayer.animation_finished
+    queue_free()