isListOfNum function
Returns true if list elements are all of type num.
Implementation
bool isListOfNum(Iterable? list) {
  if (list == null) return false;
  if (list is List<num>) return true;
  if (list.isEmpty) return false;
  if (listNotMatchesAll(list, (e) => (e is num))) return false;
  return true;
}