Add abstraction for representing tag sets

Currently, tag sets are scattered around the code base. It would be cool to have a single type for representing them and perhaps also move all of the tag sets into a single file. Tags sets include e.g.

- Singleton tags (relevant for parsing)
- Inline tags (relevant for rendering)
- Rendered tags (relevant for rendering) - perhaps declaring the complement would make more sense, i.e. non-rendered tags?
- ...