Bump handlebars from 4.3.7 to 5.1.0 (#3248)

Bumps [handlebars](https://github.com/sunng87/handlebars-rust) from
4.3.7 to 5.1.0.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/sunng87/handlebars-rust/releases">handlebars's
releases</a>.</em></p>
<blockquote>
<h2>v5.1.0</h2>
<h2>What's Changed</h2>
<ul>
<li>chore(deps-dev): bump follow-redirects from 1.15.2 to 1.15.4 in
/playground/www by <a
href="https://github.com/dependabot"><code>@​dependabot</code></a> in <a
href="https://redirect.github.com/sunng87/handlebars-rust/pull/628">sunng87/handlebars-rust#628</a></li>
<li>Add chained else expression support by <a
href="https://github.com/progmboy"><code>@​progmboy</code></a> in <a
href="https://redirect.github.com/sunng87/handlebars-rust/pull/629">sunng87/handlebars-rust#629</a></li>
</ul>
<h2>New Contributors</h2>
<ul>
<li><a href="https://github.com/progmboy"><code>@​progmboy</code></a>
made their first contribution in <a
href="https://redirect.github.com/su...