input: Write a python function to add a new key-value pair to a dictionary and return the updated dictionary.
  assert add_to_dict({"FirstName": "John"}, "LastName", "Doe") == {"FirstName": "John", "LastName": "Doe"}
  assert add_to_dict({"Age": 30}, "Country", "USA") == {"Age": 30, "Country": "USA"}
  assert add_to_dict({}, "Key", "Value") == {"Key": "Value"}
output: def add_to_dict(d, key, value):
      d[key] = value
      return d