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/diff.c

@@ -47,6 +47,7 @@
 #include "nvim/move.h"
 #include "nvim/normal.h"
 #include "nvim/option.h"
+#include "nvim/option_vars.h"
 #include "nvim/optionstr.h"
 #include "nvim/os/fs_defs.h"
 #include "nvim/os/os.h"