Add a method for appending a newline to a string if it doesn't already end with one

[jwodder]

• Possible string inputs & outputs:

  • Option 1: Take a &mut String, return nothing
  • Option 2: Take a &'a str and return Cow<'a, str>

• Possible newline parameters:

  • Option 1: A single Newline, to append if the string doesn't already end with it
  • Option 2: A Newline and a NewlineSet; the Newline is appended to the string if it doesn't already end with a newline in the NewlineSet

Cf. #7