build(deps): bump postcss from 8.5.3 to 8.5.6 (#856)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.3 to 8.5.6.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/postcss/postcss/releases">postcss's
releases</a>.</em></p>
<blockquote>
<h2>8.5.6</h2>
<ul>
<li>Fixed <code>ContainerWithChildren</code> type discriminating (by <a
href="https://github.com/Goodwine"><code>@​Goodwine</code></a>).</li>
</ul>
<h2>8.5.5</h2>
<ul>
<li>Fixed <code>package.json</code>→<code>exports</code> compatibility
with some tools (by <a
href="https://github.com/JounQin"><code>@​JounQin</code></a>).</li>
</ul>
<h2>8.5.4</h2>
<ul>
<li>Fixed Parcel compatibility issue (by <a
href="https://github.com/git-sumitchaudhary"><code>@​git-sumitchaudhary</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/postcss/postcss/blob/main/CHANGELOG.md">postcss's
changelog</a>.</em></p>
<blockquote>
<h2>8.5.6</h2>
<ul>
<li>Fixed <code>ContainerWithChildren</code> type discriminating (by <a
href="https://github.com/Goodwine"><code>@​Goodwine</code></a>).</li>
</ul>
<h2>8.5.5</h2>
<ul>
<li>Fixed <code>package.json</code>→<code>exports</code> compatibility
with some tools (by <a
href="https://github.com/JounQin"><code>@​JounQin</code></a>).</li>
</ul>
<h2>8.5.4</h2>
<ul>
<li>Fixed Parcel compatibility issue (by <a
href="https://github.com/git-sumitchaudhary"><code>@​git-sumitchaudhary</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/postcss/postcss/commit/91d6eb5c3d1ca8acb4e8e3926005acf2b066c211"><code>91d6eb5</code></a>
Release 8.5.6 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/65ffc55117bf4289b1f977986ed76fad402641b1"><code>65ffc55</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/ecd20eb7f9587d63e3f3348b768aec0e9fb000d3"><code>ecd20eb</code></a>
Fix ContainerWithChildren to allow discriminating the node type by
comparing ...</li>
<li><a
href="https://github.com/postcss/postcss/commit/c18159719e4a6d65ad7085edf1dc42e07814f683"><code>c181597</code></a>
Release 8.5.5 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/c5523fbec5f32622e77103c643e1258007c2609d"><code>c5523fb</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/2e3450c55f41e378e086f4f189e5243a573c3390"><code>2e3450c</code></a>
refactor: <code>import</code> should be listed before
<code>require</code> (<a
href="https://redirect.github.com/postcss/postcss/issues/2052">#2052</a>)</li>
<li><a
href="https://github.com/postcss/postcss/commit/4d720bd01adec2e8645bf91e725825bebb712e1b"><code>4d720bd</code></a>
Update EM text</li>
<li><a
href="https://github.com/postcss/postcss/commit/6cb4a6673fb6d8b23eb1ebe66a22b6267ab141de"><code>6cb4a66</code></a>
Release 8.5.4 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/ec5c1e031083664bed1cf91eaac72f8c61068110"><code>ec5c1e0</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/e85e9385c87499bc7e274c6ce332cf59e3988994"><code>e85e938</code></a>
Fix code format</li>
<li>Additional commits viewable in <a
href="https://github.com/postcss/postcss/compare/8.5.3...8.5.6">compare
view</a></li>
</ul>
</details>
<br />


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