fix: cast char to unsigned char in isspace call to fix char-subscripts warning

src/vm/wren_core.c

@@ -942,7 +942,7 @@ DEF_PRIMITIVE(num_fromString)
   double number = strtod(string->value, &end);
 
   // Skip past any trailing whitespace.
-  while (*end != '\0' && isspace(*end)) end++;
+  while (*end != '\0' && isspace((unsigned char) *end)) end++;
 
   if (errno == ERANGE)
   {