chore: set minimum Node.js version to 18.0.0

This also lets eslint-plugin-n flag usage of APIs unavailable in the
supported version range.

Author: watson

package.json

@@ -37,5 +37,8 @@
     "eslint-plugin-n": "^17.24.0",
     "eslint-plugin-unicorn": "^63.0.0",
     "globals": "^17.4.0"
+  },
+  "engines": {
+    "node": ">=18.0.0"
   }
 }