addNewLine method
Adds a new line to the tail of this container.
Implementation
TextBit addNewLine() {
  final tail = last ?? prev;
  if (tail is _TextNewLine) return tail..extend();
  final bit = _TextNewLine(this);
  add(bit);
  return bit;
}
    Adds a new line to the tail of this container.
TextBit addNewLine() {
  final tail = last ?? prev;
  if (tail is _TextNewLine) return tail..extend();
  final bit = _TextNewLine(this);
  add(bit);
  return bit;
}