input: Write a python function to convert a list of integers to a comma-separated string.
  assert list_to_string([1, 2, 3, 4, 5]) == "1,2,3,4,5"
  assert list_to_string([10, 20, 30, 40]) == "10,20,30,40"
  assert list_to_string([100, 200, 300]) == "100,200,300"
output: def list_to_string(lst):
      return ','.join(map(str, lst))