Nixify `treefmt.toml`

[srid]

We want to nixify treefmt.toml, and have it directly use the nix packages, in addition to having treefmt in nix shell run with the nixified configuration.

We can use https://github.com/numtide/treefmt/pull/169 but the main blocker is the lack of PRJ_ROOT (see comments).

[srid]

This is now possible using "github:srid/treefmt/withConfig-stable"

          treefmt.buildInputs = [
            (inputs'.treefmt.legacyPackages.default.withConfig {
              projectRootFile = "flake.nix";
              settings = {
                formatter = {
                  nix = {
                    command = lib.getExe pkgs.nixpkgs-fmt;
                    includes = [ "*.nix" ];
                  };
                  purescript = {
                    command = lib.getExe pkgs.nodePackages.purs-tidy;
                    options = [ "format-in-place" ];
                    # Note that given how purs-tidy is designed (takes glob, rather than explicit
                    # list of files), these patters inevitably slow down purs-tidy (thus treefmt)
                    # during the initial run.
                    includes = [
                      "src/**/*.purs"
                      "core/**/*.purs"
                      "lib/**/*.purs"
                      "src/**/*.purs"
                      "domain/**/*.purs"
                      "infrastructure/**/*.purs"
                      "ui-guide/**/*.purs"
                      "test/**/*.purs"
                    ];
                  };
                };
              };
            })
          ];

May as well retire this repo right away by moving it to upstream repo after https://github.com/numtide/treefmt/pull/169 gets merged to master.