input: Write a python function to join elements of a list with a given separator.
  assert join_list_with_separator(["apple", "banana", "cherry"], ", ") == "apple, banana, cherry"
  assert join_list_with_separator(["one", "two", "three"], "-") == "one-two-three"
  assert join_list_with_separator(["a", "b", "c"], "") == "abc"
output: def join_list_with_separator(lst, separator):
      return separator.join(lst)