feat: add support for Nix via a flake

Providing a flake would allow Nix users to checkout the codebase and immediately be able to build and iterate on the application with minimal setup.

Writing that flake is pretty straightforward and I think it would also be a good basis to provide swamp via the usual Nix Package distribution system

I, or any agent, can provide the code if you want.

And thanks for swamp!

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Automoved by swampadmin from GitHub issue #369