Code samples in the demo page

[MaxDesiatov]

I know it's going to be hard as the demo code would become self-referential that way. Maybe we could upload the demo code to release assets (it would be served through GitHub CDN then), and then the running demo would download that archive, unpack it and display each file's source code in a separate "tab"? But then there's a question of adding a new dependency for .zip support just in the demo. Maybe it's time to move the demo code into a separate repository so that we could add more dependencies to it 🤔

I would still prefer a monorepo though, the main benefit is that it's easier to add demos for in-progress and unreleased features that way.

[MaxDesiatov]

We could use something like https://github.com/JohnSundell/Splash/pull/92 for syntax highlighting.