Add MainMenu scene and script with start and quit functionality

Scripts/MainMenu.cs

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+using Godot;
+
+namespace ParasiticGod.Scripts;
+
+public partial class MainMenu : Node
+{
+    [Export] private PackedScene _gameScene;
+    [Export] private Button _startButton;
+    [Export] private Button _quitButton;
+
+    public override void _Ready()
+    {
+        if (_startButton != null)
+        {
+            _startButton.Pressed += OnStartButtonPressed;
+        }
+
+        if (_quitButton != null)
+        {
+            _quitButton.Pressed += OnQuitButtonPressed;
+        }
+    }
+
+    public override void _ExitTree()
+    {
+        if (_startButton != null)
+        {
+            _startButton.Pressed -= OnStartButtonPressed;
+        }
+
+        if (_quitButton != null)
+        {
+            _quitButton.Pressed -= OnQuitButtonPressed;
+        }
+    }
+
+    private void OnStartButtonPressed()
+    {
+        if (_gameScene == null)
+        {
+            GD.PrintErr("Game scene is not assigned in MainMenu.");
+            return;
+        }
+
+        GetTree().ChangeSceneToPacked(_gameScene);
+    }
+    
+    private void OnQuitButtonPressed()
+    {
+        GetTree().Quit();
+    }
+}

Scripts/MainMenu.cs.uid

@@ -0,0 +1 @@
+uid://cbdokimy0qarg