input: Write a python function to display a menu and allow the user to select an item or exit.
  assert menu() == "Selected item: Apple" # input sequence: '1'
  assert menu() == "Selected item: Banana" # input sequence: '3'
  assert menu() == None # input sequence: '4'
  assert menu() == "Invalid selection!" # input sequence: '5'
output: def menu():
      items = {
          '1': 'Apple',
          '2': 'Orange',
          '3': 'Banana'
      }
      print("Menu:")
      print("1. Apple")
      print("2. Orange")
      print("3. Banana")
      print("4. Exit")
  
      choice = input("Select an option: ")
  
      if choice in items:
          return f"Selected item: {items[choice]}"
      elif choice == '4':
          return None
      else:
          return "Invalid selection!"