Declare slugify as a direct dependency

config/utils.js imports slugify, but it was only resolvable because npm
hoists Eleventy's copy into the top-level node_modules. Declare it
explicitly so the build doesn't depend on that hoisting.

package-lock.json

@@ -36,6 +36,7 @@
         "postcss-import-ext-glob": "^2.1.1",
         "postcss-nested": "^6.0.0",
         "prettier": "^2.8.3",
+        "slugify": "^1.6.9",
         "tailwindcss": "^3.4.13"
       },
       "devDependencies": {