Add OpenSearch

[rnkn]

Hello. Thanks for creating this project. I use it constantly via https://openports.pl.

Solene pointed me to you after I suggested adding OpenSearch to the site. It’s a short XML file that allows searching directly from the location bar:

https://developer.mozilla.org/en-US/docs/Web/OpenSearch

Here's an example for https://openports.pl:

<OpenSearchDescription>
  <ShortName>openports.pl</ShortName>
  <Description>Search for OpenBSD ports</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Image width="16" height="16" type="image/x-icon">https://openports.pl/favicon.ico</Image>
  <Url type="text/html" template="https://openports.pl/search?pkgname={searchTerms}"/>
</OpenSearchDescription>

I'm not exactly sure how to implement this with Dancer, otherwise this would have been a PR. I assume it would go in /public but do those files go through template interpolation?

Either way, would you be interested in adding OpenSearch to the project?