feat: use treefmt-nix and split up flake.nix

flake-modules/default.nix

@@ -0,0 +1,32 @@
+{ inputs, ... }:
+{
+  imports = [
+    inputs.treefmt-nix.flakeModule
+    ./package.nix
+    ./overlay.nix
+    ./module.nix
+  ];
+
+  perSystem =
+    { pkgs, ... }:
+    {
+      # Setup formatters
+      treefmt = {
+        projectRootFile = "flake.nix";
+        programs = {
+          nixfmt.enable = true;
+          rustfmt.enable = true;
+          statix.enable = true;
+          typos.enable = true;
+        };
+      };
+
+      devShells.default = pkgs.mkShellNoCC {
+        packages = [
+          pkgs.cargo-insta
+          pkgs.cargo-udeps
+          pkgs.mold
+        ];
+      };
+    };
+}