Enable users to mark a "split" in their post, where all content beforehand is available to all readers (subject to blog / "collection" permissions) and all content after the split is unlocked only if the reader is using Web Monetization.
- Create a new shortcode, e.g. <!--premium--> that indicates where content is split
- When rendering a collection or post, hide all content after the split
- Rely on client-side JS + an Unlocking API to return everything else.
TODO: implementation details for item #3.