split method
Implementation
(BidiRope, BidiRope) split(int index) {
if (index < 0 || index > length) {
throw RangeError.range(index, 0, length, 'index');
}
final (leftRope, rightRope) = _rope.split(index);
final leftText = leftRope.toString();
final rightText = rightRope.toString();
final leftContainsRtl = BidirectionalText.containsRtl(leftText);
final rightContainsRtl = BidirectionalText.containsRtl(rightText);
return (BidiRope._(leftRope, leftContainsRtl), BidiRope._(rightRope, rightContainsRtl));
}