input: Write a function that takes a list of strings and a separator/delimiter as input, and returns the concatenation of all the elements with the specified separator.
  assert concatenate_strings(["Sample", "Text", "Hello"], ", ") == "Sample, Text, Hello"
  assert concatenate_strings(["Python", "is", "awesome"], "-") == "Python-is-awesome"
  assert concatenate_strings([], ",") == ""
  assert concatenate_strings(["SingleString"], "") == "SingleString"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)