perf: Rewriting convert_deflists for performance.

markdown/testdata/deflists/reference/hugo/04-markdown-in-def.html

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+<dl>
+<dt>Markdown Syntax</dt>
+<dd><strong>Markdown</strong> supports multiple text formatting:
+<ul>
+<li><em>Italic</em> and <strong>Bold</strong></li>
+<li><a href="https://www.markdownlang.com">Link</a></li>
+<li><code>Inline code</code></li>
+<li>
+<blockquote>
+<p>Block quote</p>
+</blockquote>
+</li>
+<li>Lists and other elements</li>
+</ul>
+</dd>
+</dl>