refactor (#6)

Reviewed-on: #6

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; }
 }