Add opt-in collapsible option to TableOfContentsExtension

[dereuromark]

What

Adds an opt-in collapsible option to TableOfContentsExtension that wraps the generated table of contents in an HTML5 <details>/<summary> disclosure, so a long contents list can start collapsed and expand on click.

Mirrors markup-carve/carve-php#297 (the carve-php line), keeping the two library families consistent.

Behavior

• Default off. With collapsible unset the output is the unchanged <nav class="toc">, so existing consumers are unaffected.
• When on, the heading list sits directly inside the disclosure (no <nav>):

<details class="toc">
<summary>Table of Contents</summary>
<ul> ... </ul>
</details>

• summary (default Table of Contents) sets the label and is HTML-escaped.
• open renders it expanded; closed by default.

Motivation

The wp-djot plugin currently produces a collapsible TOC via a plugin-level preg_replace hack that rewrites the emitted <nav class="toc"> into a <details>. This option lets the plugin drop that hack and use the library directly.

Tests

4 new cases: collapsible closed/open, custom + escaped summary, and the unchanged non-collapsible path. TOC suite green; phpstan + phpcs clean on the touched files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (df3ce69) to head (70dcd36).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #259   +/-   ##
=========================================
  Coverage     92.37%   92.37%           
- Complexity     3633     3635    +2     
=========================================
  Files           109      109           
  Lines         10277    10283    +6     
=========================================
+ Hits           9493     9499    +6     
  Misses          784      784           

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