feat: switch from flake-parts & devenv to rensa + add nixtest

nix/repo/devShells.nix

@@ -0,0 +1,29 @@
+{
+  cell,
+  inputs,
+  ...
+}: let
+  inherit (inputs) pkgs devshell treefmt;
+  inherit (cell) soonix;
+in {
+  default = devshell.mkShell {
+    imports = [soonix.devshellModule];
+    packages = [
+      pkgs.nil
+      (treefmt.mkWrapper pkgs {
+        programs = {
+          alejandra.enable = true;
+          deadnix.enable = true;
+          statix.enable = true;
+          mdformat.enable = true;
+        };
+        settings.formatter.mdformat.command = let
+          pkg = pkgs.python3.withPackages (p: [
+            p.mdformat
+            p.mdformat-mkdocs
+          ]);
+        in "${pkg}/bin/mdformat";
+      })
+    ];
+  };
+}