input: Write a function that takes a list of strings and a separator/delimiter, and returns the concatenation of all the elements 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(["A", "B", "C"], "")=="ABC"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)