Until now they were implicitly ordered by load order because of
`DescendantTracker#descendants` register them through the `inherited`
callback.

But now that on Ruby 3.1 we use the native `Class#descendants`,
the ordering is rather random, or at least can't be relied on.

So we add a counter and assign an index on every Railtie subclass
to be able to sort them explictly later.