feat: Implement a skill pickup system with a new collectable type, event handling, and dedicated pickup objects.

scripts/Resources/CollectableResource.cs

@@ -6,4 +6,9 @@ public partial class CollectableResource : Resource
 {
     [Export] public float Amount { get; set; } = 0.0f;
     [Export] public CollectableType Type { get; set; }
+
+    /// <summary>
+    /// The skill to unlock when collected. Only used when Type is Skill.
+    /// </summary>
+    [Export] public SkillData Skill { get; set; }
 }

scripts/Resources/CollectableType.cs

@@ -5,4 +5,5 @@ public enum CollectableType
     Coin,
     Kid,
     Health,
+    Skill,
 }