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