feat: add Lean language

[foxyseta]

Closes #1354.

[spenserblack]

I appreciate the creativity for this art!

While it looks fine in my terminal, it looks misaligned in GitHub. Looks like GitHub's monospace font doesn't play nicely with these characters (it improved when I forced GitHub to use my preferred monospace font). This makes me think that other monospace fonts that users might use won't have good compatibility with this art's characters. How do you feel about simplifying the art (∃ -> E, etc.)?

[foxyseta]

Right. I assume GitH*b's monospace font is lacking those Unicode characters, and so a non-monospace fallback font is used. This could indeed also happen on terminals emulators whose monospace fonts collectively do not cover such characters.

I am not just using "E" for $\exists$ and "A" for $\forall$.

[spenserblack]

Thanks!

[o2sh]

Thanks a lot for your contribution @foxyseta

I've adjusted your ASCII art to better resemble the original logo

What do you think?

    {0}       ______
    {0}|            |\              /|\       |
    {0}|            | \            / | \      |
    {0}|            |  \          /  |  \     |
    {0}|      ______|   \________/   |   \    |
    {0}|            |    \      /    |    \   |
    {0}|            |     \    /     |     \  |
    {0}|            |      \  /      |      \ |
    {0}|____________|       \/       |       \|

[foxyseta]

Love it. You're so artsy😍