Remove flake dependency on nixpkgs/core

[tcmal]

This removes the main flake's dependency on nixpkgs (later core), as discussed in auxolotl/core#7.

docs and tests was moved to a subdirectory extra.
Since using a flake in a subdirectory requires you to run nix flake update whenever the main source updates, we instead use npins and regular Nix. There might be a better way to do this I'm not aware of.

The subdirectory isn't strictly required, but it makes it obvious which parts have nixpkgs available to them.

nixfmt was also updated in the new developer environment / formatting check, in order to fix the older versions issues with doc comments. This could be changed to auxolotl/core if auxolotl/core#9 is merged.