Add a flake.nix file for building on nix

[tremby]

I don't know if you've any interest in this, but this file makes it easy for me and other nix users to get a dev shell, build, install, and run the program.

Disclaimer: I'm not a nix expert and this file may not be perfect. I've been using LLMs to help me put them together when I've needed them and this one is no exception.

A couple of things to note in case you're not familiar with nix:

• The current version number is hardcoded in this file. As far as I can tell, this is how other golang packages are done in nix too.
• There's a hash of the dependencies in there, which is meant to guarantee the build is reproducible. That'll of course have to change if dependencies are updated.