refactor: reorganize option header files (#25437)

- Move vimoption_T to option.h
- option_defs.h is for option-related types
- option_vars.h corresponds to Vim's option.h
- option_defs.h and option_vars.h don't include each other

src/nvim/cmdexpand.c

@@ -48,6 +48,7 @@
 #include "nvim/menu.h"
 #include "nvim/message.h"
 #include "nvim/option.h"
+#include "nvim/option_vars.h"
 #include "nvim/os/lang.h"
 #include "nvim/os/os.h"
 #include "nvim/path.h"