chore: add nix flake

[Kodylow]

Why

Ran as soon as I saw the announcement and was shocked to find un-nixified software.

What changed

Adds a nix flake for dev env.

Test plan

Tested locally, just run nix develop, flake can extend to add builds this just does the dev env.

Super cool to see you guys open sourcing :)

[sergeichestakov]

haha we used to 🙂 ran into some issues when we made the repo / publish step private so we decided to remove it.

thanks! a Nix expert on our team will tal soon