Bug: separator gets folded by subsection

[expikr]

0.4.37

# book.toml
output.html.fold.enable = true

<!-- SUMMARY.md -->
- [Chapter 1](./chapter_1.md)
- [Chapter 2](./chapter_2.md)
  - [Hidden Section](./hidden-section.md)

---

[Credits](./credits.md)

[ehuss]

The spacer is added to the TOC here. It's a bit tricky how that interacts with current_level. At a glance I'm not sure how that should be better organized.

[ehuss]

Note, this is a duplicate of #733.