input: Write a function to find the shortest string in a list of strings.
  assert shortest_string(["apple", "banana", "pear", "grape"]) == "pear"
  assert shortest_string(["hello", "world", "Python", "AI"]) == "AI"
  assert shortest_string(["dog", "cat", "mouse", "elephant"]) == "cat"
output: def shortest_string(strings):
      return min(strings, key=len)