input: Write a python function to count the number of lines in a multi-line string.
  assert count_lines('''This is a multi-line string\nthat spans multiple lines.''') == 2
  assert count_lines('''Hello\nWorld\nThis is a test.''') == 3
  assert count_lines('''Single line''') == 1
output: def count_lines(s: str) -> int:
      return s.count('\n') + 1