Use a nix flake instead of shell.nix

[aruZeta]

• [x] Make the actual shell.nix into a flake ~- [ ] Create a derivation to generate develop branch documentation~ ~- [ ] Create a derivation to generate develop main documentation~ ~- [ ] Create a derivation to run tests~ ~- [ ] Create a derivation to run benchmarks~

[aruZeta]

So after a bit of research and finishing the flake with nim 1.6.6 and fixed nim-unwrapped build, I think that creating derivations for running tests, benchmarks or doc generation is not the best approach, and doing nix shell -c nim/nimble command would be best to use in GitHub actions, while for local development you could use nix develop or nix shell to drop in a shell with the nim derivation.

I also have not been able to achieve creating those derivations, which is also why I'm changing the approach, if I'm able to do it I will reconsider doing it that way or not.