input: Write a python function to print the "Happy Birthday" song with a customizable name in the third line.
  assert birthday_song("Alice") == "Happy birthday to you\nHappy birthday to you\nHappy birthday dear Alice\nHappy birthday to you"
  assert birthday_song("Bob") == "Happy birthday to you\nHappy birthday to you\nHappy birthday dear Bob\nHappy birthday to you"
  assert birthday_song("Charlie") == "Happy birthday to you\nHappy birthday to you\nHappy birthday dear Charlie\nHappy birthday to you"
output: def birthday_song(name):
      return f"Happy birthday to you\nHappy birthday to you\nHappy birthday dear {name}\nHappy birthday to you"