Enh: return sub-folders when calling `list_all_files()`

[ncullen93]

Right now, calling e.g., firebase::Storage$new()$ref(some_directory)$list_files_all(response = 'some_response') returns only the list of files in that directory. It would be nice to also return the list of any sub-folders in that directory. I think this is because the custom message handler 'fireblaze-list-all-files' in storage.js only returns res.items, while the sub-folders are in a separate key called res.prefixes (see ref)

Ref: https://firebase.google.com/docs/reference/js/v8/firebase.storage.Reference#listall