input: Write a python function to filter unique strings from a list.
  assert unique_strings(["Hello", "World", "Hello", "Python"]) == ["Hello", "World", "Python"]
  assert unique_strings(["a", "b", "a", "c", "b"]) == ["a", "b", "c"]
  assert unique_strings(["apple", "banana", "apple", "orange", "banana", "pear"]) == ["apple", "banana", "orange", "pear"]
output: def unique_strings(lst):
      seen = set()
      unique_lst = []
      for string in lst:
          if string not in seen:
              unique_lst.append(string)
              seen.add(string)
      return unique_lst