Add preview for scratch buffers in buffer picker

[A-Walrus]

Resolves #3420.

The diff is quite big mostly because a big block in picker.rs is now indented more...

[A-Walrus]

would it be too on-the-nose to call it a File instead of PathOrId?

I feel like File is less descriptive, and also more confusing as people might assume it is std::fs::File or tokio::fs::File.

[archseer]

Can you resolve the conflict here? Looks good to merge otherwise

[the-mikedavis]

Thanks for working on this!