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