feat(nix): user flake instead regular shell.nix

nix/devshells.nix

@@ -0,0 +1,28 @@
+_: {
+  config.perSystem =
+    {
+      config,
+      inputs',
+      pkgs,
+      ...
+    }:
+    {
+      devShells = {
+        default = pkgs.mkShell {
+          name = "hugo-shell";
+          nativeBuildInputs = config.develPackages
+            ++ config.hugoBuildPackages
+          ;
+          shellook = ''
+            mkdir -p themes
+            ln -snf "${config.themePackage}/" "themes/mainroad"
+          '';
+        };
+        latex = pkgs.mkShell {
+          name = "latex-shell";
+          nativeBuildInputs = config.develPackages
+            ++ config.latexBuildPackages;
+        };
+      };
+    };
+}