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/ops.h

@@ -11,6 +11,7 @@
 #include "nvim/extmark.h"
 #include "nvim/macros.h"
 #include "nvim/normal.h"
+#include "nvim/option_defs.h"
 #include "nvim/os/time.h"
 #include "nvim/pos.h"
 #include "nvim/types.h"