input: Write a python function to convert a list of strings to a single CSV string.
  assert to_csv(['John', 'Joseph', 'Shirley']) == 'John,Joseph,Shirley'
  assert to_csv(['apple', 'banana', 'cherry']) == 'apple,banana,cherry'
  assert to_csv(['red', 'green', 'blue']) == 'red,green,blue'
output: def to_csv(lst):
      return ','.join(lst)