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)

[expikr]

The issue would be solved by moving the divider <li> onto the same level as chapter items rather than being nested under the immediately preceding section level.

I'd like to dig into this but can't figure out which part of the codebase populates the toc, if I can get a quick pointer then it'd greatly speed up the process.

[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.

[expikr]

Turns out the fix is as simple as just moving the divider processing to the same spot as part titles, no change to the logic whatsoever.