feat: add support for formatter

[elliott-farrall]

Since the nix flake outputs schema allows us to specify a formatter, I figured it makes sense to support formatter in snowfall-lib (without having to use outputs-builder).

The schema for formatter is similar to checks, devShells, etc but currently only allows for a single option per-system. With this in mind, I think a single derivation (dependent on system) specified at formatter/default.nix would fit well with the current snowfall-lib structure.

Since there is only one formatter, alias is not applicable here. In theory overrides could be implemented but would require setting priority between full-flake-options and user-outputs so I have skipped this for now.

The implementation of create-formatter is based on create-checks and create-shells so has some redundant steps but provides a basic proof of concept.

[jakehamilton]

Hmm, I am not totally sure if the additional abstraction is really beneficial here. The standalone formatter file would probably just be:

{ nixfmt }: nixfmt

In which case, I don't know if that is worth separating just to avoid a few lines:

outputs-builder = channels: {
  formatter = channels.nixpkgs.nxifmt;
};

[elliott-farrall]

I think you're right in that, for most people, the reduction in boilerplate is pretty minimal. For me personally, I mainly use treefmt-nix and so I might have a formatter file like:

{ pkgs
, inputs
, ...
}:

inputs.treefmt-nix.lib.mkWrapper pkgs {
  projectRootFile = ".git/config";

  settings.global.excludes = [
    "LICENSE.md"
  ];

  programs = {
    prettier.enable = true;
    taplo.enable = true;
    yamlfmt.enable = true;
  };

  settings.formatter = {
    yamlfmt.options = [ "-formatter" "retain_line_breaks_single=true" ];
  };
}

which is a bit more substantial. I do also have some check files that are one-liners, essentially pulling a check from an input and getting it into the right form. I prefer this over using outputs-builder as I feel it keeps things more organised.

I understand that adding support for formatter is probably overkill. I just thought I'd submit a PR to see if anyone else thought it was a good idea.

[TakodaS]

For posterity, I use the following configuration in snowfall-lib to set up treefmt:

outputs-builder = channels: let
    treefmtConfig = {pkgs, ...}: { 
    projectRootFile = "flake.nix";  
    programs.alejandra.enable = true; 
    # rest of config
}; 
    treefmtEval = inputs.treefmt-nix.lib.evalModule (channels.nixpkgs) treefmtConfig;
in {
    formatter = treefmtEval.config.build.wrapper;
}