(doc) add Lean language

[bryangingechen]

This is a followup to #1689. We've created a new repository at https://github.com/leanprover-community/highlightjs-lean and linked to it in SUPPORTED_LANGUAGES.md.

I'm not 100% sure we've made the new repo in the recommended way, so if a highlightjs maintainer would like to take a look and give some suggestions, I'd be very grateful!

cc: @PatrickMassot.

[joshgoebel]

It'd be nice if you ran the build script and then committed the distributable to your repo. Unless you purposely aren't wanting that?

I see you have build instructions in the readme.

[bryangingechen]

Is it useful for people to see the minified version in the repo? I generally don't commit build outputs to version control, so I'm curious to know the reasons for doing so here.

[joshgoebel]

I generally don't commit build outputs to version control, so I'm curious to know the reasons for doing so here.

Just so users desiring to use it on the web have a one click download or easy copy/paste, etc... not everyone has a build environment or the ability to set one up. Unless you're providing some other easy one-click download link?

[joshgoebel]

Is it useful for people to see the minified version in the repo?

It's not JUST minified, it's compiled specifically for auto-loading CDN-style support.

[bryangingechen]

Understood, thank you!

[joshgoebel]

Thanks for the contribution!