Switch format = "qs" to the qs2 package

c.f. https://github.com/qsbase/qs/issues/103.

We can do this back-compatibly. New targets will be saved with `qs2::qs_save()`. For existing targets, we can first try `qs2::qs_read()` and then fall back on `qs::read()`. `qs2::qs_read()` automatically detects the legacy `qs` format and throws an informative error.