input: Write a python function to extract the values of the 'string' key from a list of dictionaries.
  assert extract_strings([{'string' : 'hello world'}, {'number' : 'hello 5'}]) == ['hello world']
  assert extract_strings([{'string' : 'foo'}, {'string' : 'bar'}, {'number' : 'baz'}]) == ['foo', 'bar']
  assert extract_strings([{'number' : '123'}, {'string' : 'abc'}, {'string' : 'xyz'}]) == ['abc', 'xyz']
output: def extract_strings(dicts):
      return [d['string'] for d in dicts if 'string' in d]