Add nix support via flakes

[srid]

With this PR, it becomes possible to both develop and use tart using Nix flakes and https://github.com/srid/haskell-flake. See https://srid.ca/haskell-template for specific workflows.

[jtdaugherty]

Hi @srid, thanks for opening this. I'm a bit torn on merging it because I tend to only want to merge things that I know I will use and maintain; if I merged this, it would go unmaintained and then I'm concerned I'd get bug reports or requests to fix it, and I wouldn't be in a position to do that. Is there any way this functionality could be kept somewhere outside the repo and still be useful? If that were possible, I'd be happy to link to a third-party repo from the README about it, for example.

[srid]

Is there any way this functionality could be kept somewhere outside the repo and still be useful? If that were possible, I'd be happy to link to a third-party repo from the README about it, for example.

You could say something to the effect of:

In order to develop and/or install this project using Nix, you can add a `flake.nix` based on [haskell-flake](https://github.com/srid/haskell-flake).

Because that's what I essentially did in this PR.

[srid]

Closing in favour of #19