flake: enable treefmt for formatting

- for now, uses only `nixfmt-rfc-style`
- can be invoked with `nix flake check` and `nix fmt`

flake.lock

@@ -307,6 +307,7 @@
         "nixos-unstable": "nixos-unstable",
         "nixpkgs": "nixpkgs_2",
         "nur": "nur",
+        "treefmt-nix": "treefmt-nix",
         "utils": "utils"
       }
     },
@@ -365,6 +366,26 @@
         "type": "github"
       }
     },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1710278050,
+        "narHash": "sha256-Oc6BP7soXqb8itlHI8UKkdf3V9GeJpa1S39SR5+HJys=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "35791f76524086ab4b785a33e4abbedfda64bd22",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
+    },
     "utils": {
       "inputs": {
         "systems": "systems_2"