flake: add wildmidi in devshell

flake.nix

@@ -39,5 +39,12 @@
       in {
         indigo-night = makeSheetMusic "indigo-night";
       };
+
+      devShell = pkgs.mkShellNoCC {
+        buildInputs = [
+          pkgs.lilypond-with-fonts
+          pkgs.wildmidi
+        ];
+      };
     });
 }