input: Write a python function to sort a list of strings based on their length.
  assert sort_strings(['banana', 'apple', 'mango', 'pear']) == ['pear', 'apple', 'mango', 'banana']
  assert sort_strings(['a', 'abcd', 'abc', 'ab']) == ['a', 'ab', 'abc', 'abcd']
  assert sort_strings(['longest', 'long', 'longer']) == ['long', 'longer', 'longest']
output: def sort_strings(strings):
      return sorted(strings, key=len)