Support singleton items

[rosszurowski]

I recently migrated from contentlayer to content-collections. (Side note: thanks for your work building this, it's much simpler and easier to use!) One thing that contentlayer supports is singleton collections, that only represent a single document. This is really useful for global config like structured documentation navigation data, a global authors list, or other global config.

It's easy to work around right now by just taking the first item in a collection and throwing an error if not set, but it'd be nice if content-collections could verify this for me at compile time instead.