feat: switch to nixfmt and add pre-commit hook

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+repos:
+  - repo: https://github.com/nixos/nixfmt
+    rev: v1.2.0
+    hooks:
+      - id: nixfmt-nix
+        args: [--, --check]

flake.nix

@@ -94,11 +94,22 @@
         lib = nixpkgs.lib;
       } { };
     }
-    // flake-utils.lib.eachDefaultSystem (system: {
+    // flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
         packages = import ./packages {
           inherit my-lib;
           lib = nixpkgs.lib;
         } (inputs // { inherit system; });
-      formatter = nixpkgs.legacyPackages.${system}.nixfmt-rfc-style;
-    });
+
+        formatter = pkgs.nixfmt-tree;
+
+        devShells.default = pkgs.mkShell {
+          packages = [ pkgs.pre-commit ];
+        };
+      }
+    );
 }

treefmt.toml

@@ -1,4 +0,0 @@
-[formatter.nix]
-command = "nix"
-options = ["fmt"]
-includes = [ "*.nix" ]