Add marp dev shell

Signed-off-by: Oliver Davies <oliver@oliverdavies.uk>

flake.nix

@@ -122,13 +122,9 @@
       };
     in
     {
-      devShells.${system}.default =
-        with pkgs;
-        mkShell {
-          packages =
-            with pkgs;
-            commonBuildInputs
-            ++ [
+      devShells.${system} = with pkgs; {
+        default = mkShell {
+          packages = commonBuildInputs ++ [
             ghostscript
             just
             pdfpc
@@ -136,9 +132,15 @@
           ];
         };
 
+        marp = mkShell {
+          packages = [ marp-cli ];
+        };
+      };
+
       packages.${system} = {
         inherit shared-assets;
-      } // talks;
+      }
+      // talks;
 
       formatter.${system} = pkgs.nixfmt-rfc-style;
     };