extend method
Returns the smallest range that contains both this range and the given
position.
Implementation
@override
BufferRangeLine extend(CellOffset position) {
final self = normalized;
final begin = self.begin.isAfter(position) ? position : self.begin;
final end = self.end.isBefore(position) ? position : self.end;
return BufferRangeLine(begin, end);
}