Allow to have a Handle<LoadedFolder>

[Selene-Amanita]

bevy_fluent expects a Handle<LoadedFolder> to work, but bevy_asset_loader doesn't allow to have one directly.

It is possible to get around it: kgv/bevy_fluent#43 (comment) but it is tedious.

bevy_fluent should accept a Vec<UntypedHandle> in the future, but other libraries might want a Handle<LoadedFolder>, it would be convenient for bevy_asset_loader to offer it for collections.