New package: Lean

[gebner]

This PR adds a package for the Unicode abbreviations used by the editor plugins for the Lean theorem prover.

[federico-terzi]

Hey @gebner ,

Looks great! Thank you for the help :) Could you please pull from the latest main and repush here? Otherwise the CI won't be able to run correctly

Thanks! :)

[smeech]

Hey @gebner ,

Looks great! Thank you for the help :) Could you please pull from the latest main and repush here? Otherwise the CI won't be able to run correctly

Thanks! :)

No response to @federico-terzi's request 2½ years ago.
This is one of two Lean PRs, the other being the slightly broken #107.
No cmd, script or shell on search, but over 4,000 expansions, mainly with hex-code representation of obscure characters.
Interesting reversal of the conventional - trigger:/replace: pairs - I was interested to see this still works.

[AucaCoyan]

I gave a quick glance over a few lines and it seems alright.
I hope I don't break anything by quoting the infamous LGTM (looks good to me). We can revert if any issues are raised 👍🏼 .
Thank you Smeech and @gebner !