refactor: move some constants out of vim_defs.h (#26298)

src/nvim/ex_docmd.h

@@ -28,6 +28,8 @@ enum {
 // Whether a command index indicates a user command.
 #define IS_USER_CMDIDX(idx) ((int)(idx) < 0)
 
+enum { DIALOG_MSG_SIZE = 1000, };  ///< buffer size for dialog_msg()
+
 /// Structure used to save the current state.  Used when executing Normal mode
 /// commands while in any other mode.
 typedef struct {