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.json

@@ -47,6 +47,7 @@
     "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": {