refactor: Enable unreachable cleanup logic in main()

> refactor `main_implemntation()` cleanup to run once on `launcher::shutdown()`, not sure why it was defined like that (unreachable code, at least for iidx)
> not thread safe, but probably not needed (no obvious failures as well)
> related to #1

launcher/shutdown.h

@@ -2,8 +2,10 @@
 
 #include <windows.h>
 #include <stdlib.h>
+#include <functional>
 
 namespace launcher {
+    extern std::function<void()> on_shutdown;
     void stop_subsystems();
     void kill(UINT exit_code = EXIT_FAILURE);
     void shutdown(UINT exit_code = EXIT_SUCCESS);