Did I miss license or it doesn't exist in zip file?

There is no license file yet. I'll add it

I'd appreciate dual-license because of https://opensource.org/blog/public-domain-is-not-open-source etc.

Thanks for pointing this out! I changed the license.

By the way, if you plan to modify it, we can do it collaboratively. Feel free to suggest options - I am open to different approaches

By the way, if you plan to modify it, we can do it collaboratively.

Sure, I could optimize first (which will be with drastic changes) with a repository on Github, and then I'll see what I could do next.

However, I got used to making it separately, for instance: https://mods.factorio.com/mod/m-rpg_items (although it wasn't collaborative)