remove trigger markup for non-scrolly formats

[andrewpbray]

It'd be nice if a closeread doc could "fail gracefully" when rendered to a different format like pdf or html. Right now it does so fairly well with the exception of the @cr-mysticky which remain visible in the narrative blocks after rendering. A few options:

1. Appeal to Quarto devs to add cr- as a protected prefix. Then we could have Quarto scrub them.
2. Switch closeread to a filter that could be run even on different formats. When run on those formats, all it'd run is a trigger scrubber.
3. Add options to closeread-html to also produce an alternate (non-scrolly) format.

[jimjam-slam]

Mmmm, good idea! I still love the idea of getting Closeread all the way down to a filter, but I can't remember now what our blocker on that was.

[andrewpbray]

Haha, I was trying to remember why we decided to hold off on the filter for now and ... I really couldn't remember our rationale for sticking with a format for the time being!