input: Write a function to calculate the sum of a list of numbers using a while loop.
  assert add_numbers([1, 2, 3, 4, 5]) == 15
  assert add_numbers([10, 20, 30, 40]) == 100
  assert add_numbers([0, -1, 1, -2, 2]) == 0
output: def add_numbers(numbers):
      total = 0
      index = 0
      while index < len(numbers):
          total += numbers[index]
          index += 1
      return total