refactor: use bool to represent boolean values

src/nvim/eval/vars.c

@@ -616,7 +616,7 @@ static void list_tab_vars(int *first)
 /// List variables in "arg".
 static const char *list_arg_vars(exarg_T *eap, const char *arg, int *first)
 {
-  int error = false;
+  bool error = false;
   int len;
   const char *name;
   const char *name_start;