input: Write a function to add multiple items to a dictionary for a given key.
  assert add_multiple_items({'a': [1, 2]}, 'a', [3, 4]) == {'a': [3, 4]}
  assert add_multiple_items({}, 'b', [5, 6, 7]) == {'b': [5, 6, 7]}
  assert add_multiple_items({'x': [9]}, 'y', [1, 2, 3]) == {'x': [9], 'y': [1, 2, 3]}
output: def add_multiple_items(dct, key, items):
      dct[key] = items
      return dct