Add flake.nix

[expede]

This PR is extremely optional, but I figured I'd contribute this while I had the hood up for #32

In short: I added a flake.nix for any other Nix users show up. This doesn't impact non-Nix users in any way, but gives anyone with Nix installed an instant, correctly configured dev environment.

[vmx]

There used to be a flake.nix file in other IPLD/Multiformat repos I maintain. My problem is that I know almost nothing about Nix and to me it seems like the lock file would also needed to be updated from time to time.

I can neither really review the change, not maintain it. Hence I would prefer if we wouldn't need to check Nix specific things into this repo. If there's any other way to make the life of Nix users easier, I'm happy to merge any PRs in regards to that.

[expede]

@vmx Okay no prob 👍

If there's any other way to make the life of Nix users easier, I'm happy to merge any PRs in regards to that.

It's pretty much just the flake.nix. Being open source, Nix people would need to contribute updates to this repo to help each other out (like this PR), but it sounds like that's not how you want to run this repo which is totally fine!