input: Write a python function to convert a JSON string to a dictionary.
  assert json_to_dict('{"key": "value"}') == {"key": "value"}
  assert json_to_dict('{"name": "John", "age": 30}') == {"name": "John", "age": 30}
  assert json_to_dict('{"city": "New York", "population": 8000000}') == {"city": "New York", "population": 8000000}
output: import json
  
  def json_to_dict(json_string):
      return json.loads(json_string)