linkTo method
Link a directed link, if from
and to
don't exist, they will be added
Optional tags
for setting tags
Implementation
void linkTo(from, to, {List tags = const []}) {
final _f = _map_add_or_get(from, _newNode);
final _t = _map_add_or_get(to, _newNode);
_t.setFrom(_f);
_f.setTo(_t);
_f.setTag(_t, tags);
}