Simpler approach for storing and accessing document settings

[lionel-]

Follow up on #122.

• We currently have a setting resolver that caches settings for the folders in the workspace.

• We don't store resolved settings in our Document objects to avoid unnecessary copies of data. Nor do we store references because we'd have to deal with lifetimes and invalidation.

That might be a good use case for trying out caching with Salsa. We could then simply store the settings in every document since they'd be interned and cached by Salsa, which would also handle invalidation. We'd spare a bunch of code and have better ergonomics.