fix: set tag types for TOC properly in PDF exports

[greatislander]

See pressbooks/private#2099.

To test:

1. Copy these files into pressbooks-book/packages/buckram.
2. Export a PDF.
3. Inspect tags in Acrobat Pro.

[MichSpieler]

@arzola Ned has asked for a review here, can you do that? It would fix an accessibility issue that a client reported.

[arzola]

I'll take a look to see if we can squeeze this in our current sprint(if not next sprint will be handled)

[SteelWagstaff]

This looks terrific and just like what we need. Thanks @greatislander!

[greatislander]

@arzola can we get a review of this? It ideally should have been in that release.

[MichSpieler]

sub-issue addressed by this fix: https://github.com/pressbooks/private/issues/2392