chore(package): add engines floor for Node >=20

nerror has no committed lockfile, so installs can pull transitive
deps that require newer Node (e.g. brace-expansion@5) and fail
unpredictably. Declaring an engines floor surfaces this explicitly.

This feels reasonable as nerror 2.0.0 is not expected to be used
or work with node versions older than 20.

Author: mason-albert

package.json

@@ -9,6 +9,9 @@
     "url": "https://github.com/Netflix/nerror"
   },
   "license": "MIT",
+  "engines": {
+    "node": ">=20"
+  },
   "files": [
     "lib",
     "lib/index.d.ts"