Bump shiki from 1.6.3 to 1.6.4 (#5908)

Bumps [shiki](https://github.com/shikijs/shiki/tree/HEAD/packages/shiki)
from 1.6.3 to 1.6.4.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/shikijs/shiki/releases">shiki's
releases</a>.</em></p>
<blockquote>
<h2>v1.6.4</h2>
<h3>   🚀 Features</h3>
<ul>
<li>Add CSS variables for markup diff tokens  -  by <a
href="https://github.com/SamyPesse"><code>@​SamyPesse</code></a> in <a
href="https://redirect.github.com/shikijs/shiki/issues/698">shikijs/shiki#698</a>
<a href="https://github.com/shikijs/shiki/commit/84c0b539"><!-- raw HTML
omitted -->(84c0b)<!-- raw HTML omitted --></a></li>
<li><strong>rehype</strong>: Support default language option  -  by <a
href="https://github.com/fuma-nama"><code>@​fuma-nama</code></a> and <a
href="https://github.com/antfu"><code>@​antfu</code></a> in <a
href="https://redirect.github.com/shikijs/shiki/issues/692">shikijs/shiki#692</a>
<a href="https://github.com/shikijs/shiki/commit/724b410e"><!-- raw HTML
omitted -->(724b4)<!-- raw HTML omitted --></a></li>
</ul>
<h5>    <a
href="https://github.com/shikijs/shiki/compare/v1.6.3...v1.6.4">View
changes on GitHub</a></h5>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/shikijs/shiki/commit/2469296ccebad1eef8d4a14716ec3aa55591372c"><code>2469296</code></a>
chore: release v1.6.4</li>
<li><a
href="https://github.com/shikijs/shiki/commit/1f056f52c12c0a9f9003b3b98a5c6f7f458e57e2"><code>1f056f5</code></a>
chore: update deps</li>
<li><a
href="https://github.com/shikijs/shiki/commit/84c0b5397cb5c2a5cf3934bb4c0254965c56ab8f"><code>84c0b53</code></a>
feat: add CSS variables for markup diff tokens (<a
href="https://github.com/shikijs/shiki/tree/HEAD/packages/shiki/issues/698">#698</a>)</li>
<li>See full diff in <a
href="https://github.com/shikijs/shiki/commits/v1.6.4/packages/shiki">compare
view</a></li>
</ul>
</details>
<br />


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package.json

@@ -37,6 +37,6 @@
     "markdown-it-deflist": "^3.0.0",
     "markdown-it-table": "^4.1.1",
     "sass": "^1.77.4",
-    "shiki": "^1.6.3"
+    "shiki": "^1.6.4"
   }
 }