input: Write a python function to sort a list of strings ignoring case.
  assert sort_strings(["Apple", "banana", "APple"]) == ["APple", "Apple", "banana"]
  assert sort_strings(["Zebra", "apple", "elephant", "Apple"]) == ["Apple", "apple", "elephant", "Zebra"]
  assert sort_strings(["Banana", "apple", "APPLE"]) == ["APPLE", "apple", "Banana"]
output: def sort_strings(strings):
      return sorted(strings, key=lambda s: s.lower())