Add Syntax Highlighting

[zephraph]

Uses shiki for syntax highlighting. It's not at all doing it how you might expect though. Syntax highlighting is actually performed on val.town via my highlight val.

The process is something like this:

1. Render the markdown to html
2. In the markdown middleware, after the response is prepared run cf's HTMLRewriter to find any code blocks that need to be highlighted
   • Hash the source of the codeblock along with the theme and language
   • If that has is in KV, swap it with the html there and return the response
   • Otherwise call out to the highlight val to get the highlighted HTML
   • Write that html to KV (wait until complete, but don't block)
   • Return the result

It's a little complicated from the outset, but I wanted to show off the ability to compose site features that can be edited externally without recompiling the site. An improvement to be made here is when the val itself changes it should bust the cache somehow. Low priority for me, but would be fun.