Added configurable weapons.

AlientAttack.MonoGame/Things/Bullets/BulletDef.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace AlienAttack.MonoGame.Things.Bullets;
+
+public sealed class BulletDef
+{
+    public required BulletKind Kind { get; init; }
+    public required int Width { get; init; }
+    public required int Height { get; init; }
+    public required Func<float, float, float, float, Sprite, Bullet> Create { get; init; }
+}