input: Write a python function to join a list of strings with a given separator.
  assert join_strings(["Python", "is", "a", "programming", "language"], " ") == "Python is a programming language"
  assert join_strings(["apple", "banana", "cherry"], ", ") == "apple, banana, cherry"
  assert join_strings(["one", "two", "three"], "-") == "one-two-three"
output: def join_strings(strings, separator):
      return separator.join(strings)