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/runtime.h

@@ -9,6 +9,7 @@
 #include "nvim/ex_cmds_defs.h"
 #include "nvim/ex_eval_defs.h"
 #include "nvim/garray.h"
+#include "nvim/option_defs.h"
 #include "nvim/pos.h"
 #include "nvim/types.h"