add LdtkFieldInstance and LdtkEntityMap classes; update LdtkSceneBuilder to support entity mapping

addons/csharp_ldtk_importer/LdtkEntityMap.cs

@@ -0,0 +1,11 @@
+using Godot;
+using Godot.Collections;
+
+namespace CSharpLdtkImporter;
+
+[Tool]
+[GlobalClass]
+public partial class LdtkEntityMap : Resource
+{
+    [Export] public Dictionary<string, PackedScene> EntitySceneMap { get; set; } = new();
+}