added nix dev environment

[arichtman]

basically a copy of https://github.com/the-nix-way/dev-templates

For anyone else like me panicking at getting a working environment setup last minute 😅

[andrewbanchich]

I already make pre-compiled binaries available in the Releases so there is no need for Shreddit users to know what Rust even is. For setting up a Rust dev env on Nix, that's something in the domain of Rust / Nix, not Shreddit.

Thanks though!