useCollection add does not work offline, and does not return doc IDs

[mattiaseyram]

I'm attempting to use swr-firestore instead of any direct firebase library calls and noticed adding multiple documents does not return their new IDs. After looking at the source, also noticed that the useCollection add won't work if the user is in offline mode.

[nandorojo]

Interesting. I think this makes sense for offline mode. I'd have to think about the API change for the returning the docs, but at first glance I think your PR is consistent with expected behavior.

The one problem is that you don't really know if the batch fails or anything. At which point I'm left wondering if this should be handled by the user or not. I like the idea of magically solving issues, but it does feel a bit like an anti pattern when it comes to catching errors, so I'll think about that.