input: Write a python function to greet a user using their name provided as a command line argument.
  assert greet_user('Alice') == "Greetings, Alice!"
  assert greet_user('Bob') == "Greetings, Bob!"
  assert greet_user('Charlie') == "Greetings, Charlie!"
output: def greet_user(name):
      return f"Greetings, {name}!"