input: Write a python function to concatenate two lists of strings with a separator and return the resulting string.
  assert concatenate_lists_with_separator(["Hello", "World"], ["Python", "Programming"], ",") == "Hello,World,Python,Programming"
  assert concatenate_lists_with_separator(["One", "Two"], ["Three", "Four"], " ") == "One Two Three Four"
  assert concatenate_lists_with_separator([], ["Only", "Second"], "-") == "Only-Second"
output: def concatenate_lists_with_separator(list1, list2, separator):
      combined_list = list1 + list2
      return separator.join(combined_list)