input: Write a function that takes a list of strings and a separator/delimiter as input. The function should return the concatenation of all the elements in the list using the given separator.
  assert concatenate_strings(["Sample","Text","Hello"], "-")=="Sample-Text-Hello"
  assert concatenate_strings(["Python","is","fun"], " ")=="Python is fun"
  assert concatenate_strings(["One","Two","Three"], "+")=="One+Two+Three"
  assert concatenate_strings([], "-")==""
  assert concatenate_strings(["Single"], "-")=="Single"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)