rename ivmr.c to interface.c

dynamically allocate interface memory.

remove global var iVMR

fix duplicate error code for VBVMR_Logout and VBVMR_RunVoicemeeter

include/interface.h

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) 2024 Onyx and Iris
+ *
+ * This library is free software; you can redistribute it and/or modify it
+ * under the terms of the MIT license. See `ivmr.c` for details.
+ */
+
+#ifndef __IVMR_H__
+#define __IVMR_H__
+
+#include "VoicemeeterRemote.h"
+
+PT_VMR create_interface();
+
+#endif /* __IVMR_H__ */