input: Write a python function to convert a list of strings into a single string separated by commas.
  assert list_to_string(["apple", "banana", "cherry"]) == "apple,banana,cherry"
  assert list_to_string(["hello", "world"]) == "hello,world"
  assert list_to_string([]) == ""
output: def list_to_string(lst):
      return ",".join(lst)