input: Write a function that takes a list of strings and returns the concatenation of all the elements with a specified separator. The function should join the elements of the list using the provided separator.
  
  assert concatenate_strings(["Sample", "Text", "Hello"], " / ") == "Sample / Text / Hello"
  assert concatenate_strings(["Python", "Code"], " | ") == "Python | Code"
  assert concatenate_strings(["Data", "Science"], " - ") == "Data - Science"
  assert concatenate_strings([], " & ") == ""
  assert concatenate_strings(["Single"], "-") == "Single"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)