input: Write a python function to split a comma-separated string into a list of strings.
  assert split_string("Ruby,Python,JavaScript") == ["Ruby", "Python", "JavaScript"]
  assert split_string("Java,C++,C#,Go") == ["Java", "C++", "C#", "Go"]
  assert split_string("HTML,CSS,JavaScript,SQL") == ["HTML", "CSS", "JavaScript", "SQL"]
output: def split_string(s):
      return s.split(',')