metaformats: for home pages (ie path /), consider also extracting link rel=icon, etc

[snarfed]

There's a range of newer alternatives to favicon.ico for indicating an image or icon that represents a web site: link rel=icon, link rel=apple-touch-icon, meta name=msapplication-TileImage, etc. Not sure how many of these we want to dig into, but it might be nice to support at least some of the more established standard ones like link rel=icon.

As an example, https://thecozy.cat/ signed up for Bridgy Fed recently, and has no representative h-card or meta tags that metaformats currently finds, but it has this in its head:

<link rel="icon" href="https://thecozy.cat/wp-content/uploads/2023/08/cropped-The-Cozy-Cat-Logos-AUtumn-32x32.png" sizes="32x32" />
<link rel="icon" href="https://thecozy.cat/wp-content/uploads/2023/08/cropped-The-Cozy-Cat-Logos-AUtumn-192x192.png" sizes="192x192" />
<link rel="apple-touch-icon" href="https://thecozy.cat/wp-content/uploads/2023/08/cropped-The-Cozy-Cat-Logos-AUtumn-180x180.png" />
<meta name="msapplication-TileImage" content="https://thecozy.cat/wp-content/uploads/2023/08/cropped-The-Cozy-Cat-Logos-AUtumn-270x270.png" />

[snarfed]

^ that commit (snarfed/webutil@1f162c5) has an example implementation that I'm now using in Bridgy Fed.