move method
Moves the page at from to position to in the final list.
Implementation
void move(int from, int to) {
_checkIndex(from, 'from');
_checkIndex(to, 'to');
if (from == to) return;
final before = List<PdfPage>.of(_pages);
final page = _pages.removeAt(from);
_pages.insert(to, page);
_afterMutation(before);
}