input: Write a python function to sort a list of strings case-insensitively.
  assert sort_strings(['apple', 'Orange', 'pear']) == ['apple', 'Orange', 'pear']
  assert sort_strings(['Banana', 'apple', 'orange']) == ['apple', 'Banana', 'orange']
  assert sort_strings(['zebra', 'Monkey', 'apple']) == ['apple', 'Monkey', 'zebra']
output: def sort_strings(strings):
      return sorted(strings, key=str.lower)