feat: engine class impl + IApplication interface

include/app/app.h

@@ -0,0 +1,19 @@
+#ifndef APPLICATION_H_
+#define APPLICATION_H_
+
+#include "window/event.h"
+
+class IApplication {
+public:
+    virtual ~IApplication() = default;
+
+    virtual void OnInit() {};
+    virtual void OnUpdate() {};
+    virtual void OnRender() {};
+    virtual void OnShutdown() {};
+    
+    virtual void OnEvent() {};
+    virtual void OnWindowResized(const WindowResized& e) {};
+};
+
+#endif // APPLICATION_H_