Добавил главное меню

Assets/Scenes/MainMenu.unity.meta

@@ -0,0 +1,7 @@
+fileFormatVersion: 2
+guid: 8a5dc7b7de898674b8e1ce57e6fef727
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Assets/Scripts/UI/MainMenu.cs

@@ -0,0 +1,13 @@
+using UnityEngine;
+using UnityEngine.SceneManagement;
+
+public class MainMenu : MonoBehaviour
+{
+    public void StartGame()
+    {
+        SceneManager.LoadScene("Island");
+    }
+
+    public void SourceCodeLinkClick() =>
+        Application.OpenURL("https://github.com/ryzij/Cube-Tower-Defense");
+}

Assets/Scripts/UI/MainMenu.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: 1995f0bece1e8b84bbd28677df99ade9

ProjectSettings/EditorBuildSettings.asset

@@ -5,6 +5,9 @@ EditorBuildSettings:
   m_ObjectHideFlags: 0
   serializedVersion: 2
   m_Scenes:
+  - enabled: 1
+    path: Assets/Scenes/MainMenu.unity
+    guid: 8a5dc7b7de898674b8e1ce57e6fef727
   - enabled: 1
     path: Assets/Scenes/Island.unity
     guid: 99c9720ab356a0642a771bea13969a05