Support highlighting for Lean language

Hello, thank you a lot for pandoc & this package, I'm using a lot of Lean code in my presentations and it'd be awesome to have support for it using pandoc.

Here's a link: https://leanprover.github.io/

There is highlighting in Pygments, VSCode, and many other tools, so I'm not sure how hard is it to extract a KDE definition for this. If it's not too hard, I can PR it myself.