input: Write a python function to print a square of given size using stars.
  assert print_square(3) == "****\n****\n****\n"
  assert print_square(5) == "*****\n*****\n*****\n*****\n*****\n"
  assert print_square(2) == "**\n**\n"
output: def print_square(size):
      square = ''
      for _ in range(size):
          square += '*' * size + '\n'
      return square