input: Write a python function to create a Timer class that counts down from a specified duration and prints the remaining minutes and seconds every second until the timer is completed.
  assert Timer(5).start() == "Timer completed"  # Test for a 5-second timer
  assert Timer(10).start() == "Timer completed"  # Test for a 10-second timer
  assert Timer(0).start() == "Timer completed"  # Test for a 0-second timer
output: import time
  
  class Timer:
      def __init__(self, duration):
          self.duration = duration
  
      def start(self):
          total_seconds = self.duration
          while total_seconds > 0:
              minutes, seconds = divmod(total_seconds, 60)
              print(f"{minutes:02d}:{seconds:02d}")
              time.sleep(1)
              total_seconds -= 1
          print("Timer completed")
          return "Timer completed"