added fullscreen

src/Main.cs

@@ -0,0 +1,23 @@
+using Godot;
+using System;
+
+public class Main : Node2D
+{
+    // Declare member variables here. Examples:
+    // private int a = 2;
+    // private string b = "text";
+
+    // Called when the node enters the scene tree for the first time.
+    public override void _Ready()
+    {
+        
+    }
+
+  public override async void _Process(float delta)
+  {
+        if (Input.IsActionJustReleased("fullscreen"))
+        {
+            OS.WindowFullscreen = !OS.WindowFullscreen;
+        }
+  }
+}