movePrevious method
Move back to the previous element.
Returns true and updates current if successful.
Implementation
bool movePrevious() {
assert(isValid);
if (_currentLine <= 0) {
return false;
}
final MapEntry<Offset, TextPosition> position = _getTextPositionForLine(_currentLine - 1);
_currentLine -= 1;
_currentOffset = position.key;
_currentTextPosition = position.value;
return true;
}