Overrides the newline definition set when creating a new [type@GLib.
Regex], any Unicode newline sequence is recognised as a newline. These are `\r`, `\n` and `\r\n`, and the single characters U+000B LINE TABULATION, U+000C FORM FEED (FF), U+0085 NEXT LINE (NEL), U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR.