Shiki syntax highlight support

## Feature request

#### Problem or desire

Prism syntax highlight is quite bad (highlight.js is bad too).

#### Proposal

Support [Shiki](http://shiki.style/).

[Shiki](https://github.com/shikijs/shiki) has better syntax highlight because it uses TextMate grammar instead of shady regexp. It uses the same engine as in VS Code.

#### Implementation

I guess the only thing to do on doscify side, is to add a config option that allows to generate a different highlighter class prefix. That way all JS syntax highlighter could nearly be supported.

- shiki: css class `shiki`
- prism: css class `language-` prefix
- highlightjs: css class `hljs`

https://shiki.style/guide/install#cdn-usage