input: Write a python function to find the longest string in a list.
  assert find_longest_string(['apple', 'banana', 'cherry', 'date']) == 'banana'
  assert find_longest_string(['hello', 'world', 'python', 'programming']) == 'programming'
  assert find_longest_string(['short', 'longer', 'longest']) == 'longest'
output: def find_longest_string(strings):
      return max(strings, key=len)