Replace Prism code highlighter with Shiki

[lachlancollins]

Problem

The Prism code highlighter is quite limited for our use case, where we are commonly rendering languages like Vue or Svelte. Even JSX highlighting is often poor - see below:

Shiki is a modern alternative which uses the same engine as VS Code, meaning that how it looks on the site will be how it looks in VS Code. Unfortunately, it is ESM-only which has been challenging to implement in Remix.

Potential Solution

rehype-pretty-code - uses Shiki, hooks into unified/remark/rehype ecosystem (used by react-markdown).