Improve favicon support, in particular for light/dark mode

[chalin]

Possible improvements:

• Support favicons at the site root
• Also support icon files being at the root
• Use with to avoid generating links to files that DNE
• Report a (suppressible) warning if no favicon is found
• Support light/dark mode ico files maybe something like this:

  <link rel="icon" href="/favicons/favicon.ico" media="(prefers-color-scheme: light)">
  <link rel="icon" href="/favicons/favicon-dark.ico" media="(prefers-color-scheme: dark)">