various updates

games/space invaders/invaders_bullet.gd

@@ -0,0 +1,12 @@
+extends Area2D
+
+@export var speed : float  = 100
+
+# Called when the node enters the scene tree for the first time.
+func _ready() -> void:
+	pass # Replace with function body.
+
+
+# Called every frame. 'delta' is the elapsed time since the previous frame.
+func _process(delta: float) -> void:
+	position.y -= delta * speed