There is no need to detect the format and then the version when both can be
done at the same time.
Add a benchmark as well, on large and small atom and rss files.
- Surface `localizedError` in FindSubscriptionsFromWellKnownURLs via slog
- Use an inline declaration for new subscriptions, like done elsewhere in the
file, if only for consistency's sake
- Preallocate the `subscriptions` slice when using an RSS-bridge,
it's a good practise, and it might even marginally improve
performances when adding __a lot__ of feeds via an rss-bridge instance, wooo!
