All <nav epub:type="…"> elements in EPUB navigation doc should be shown in TOC box

[MurakamiShinyu]

A sample EPUB:
https://github.com/IDPF/epub3-samples/tree/main/30/indexing-for-eds-and-auths-3f

There are 4 <nav epub:type="…"> elements in this EPUB's navigation document:
https://github.com/IDPF/epub3-samples/blob/main/30/indexing-for-eds-and-auths-3f/EPUB/nav.xhtml

<nav epub:type="toc">
  <h2>Contents</h2>
  …
</nav>
<nav epub:type="landmarks">
  <h2>Landmarks</h2>
  …
</nav>
<nav epub:type="index-term-categories" hidden="">
  <h1>Index Term Categories</h1>
  …
</nav>
<nav epub:type="page-list" hidden="">
  <h2>Page Navigation</h2>
  …
</nav>

Test with Vivliostyle Viewer v2.27.0:
https://vivliostyle.github.io/viewer/v2.27.0/#src=https://github.com/IDPF/epub3-samples/tree/main/30/indexing-for-eds-and-auths-3f

Screenshot:

The "Contents", "Landmarks", and "Page Navigation" are shown but the "Index Term Categories" is not.

These all <nav epub:type="…"> elements should be shown in the TOC box. The EPUB spec says:

EPUB navigation documents MAY contain one or more nav elements in addition to the toc, page-list, and landmarks nav elements defined in the preceding sections. If these nav elements are intended for reading system processing, they MUST have an epub:type attribute and are subject to the content model restrictions defined in 7.3 The nav element: restrictions.

(from https://www.w3.org/TR/epub-33/#sec-nav-def-types-other)

The hidden attribute has no effect on how reading systems render the navigation data outside of the content flow (such as in dedicated navigation user interfaces provided by reading systems).

(from https://www.w3.org/TR/epub-33/#sec-nav-doc-use-spine)

[MurakamiShinyu]

Fixed in #1271.

Test: https://vivliostyle-git-fix-issue1270-vivliostyle.vercel.app/#src=https://github.com/IDPF/epub3-samples/tree/main/30/indexing-for-eds-and-auths-3f

Screenshot: