doc: allow the $schema property in node.config.json

The documentation states people can use the `$schema` property, but the
JSON schema forbids this. This change allows it.

PR-URL: https://github.com/nodejs/node/pull/57560
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Jordan Harband <ljharb@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Pietro Marchini <pietro.marchini94@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/node-config-schema.json

@@ -2,6 +2,9 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "additionalProperties": false,
   "properties": {
+    "$schema": {
+      "type": "string"
+    },
     "nodeOptions": {
       "additionalProperties": false,
       "properties": {

lib/internal/options.js

@@ -45,6 +45,10 @@ function generateConfigJsonSchema() {
     $schema: 'https://json-schema.org/draft/2020-12/schema',
     additionalProperties: false,
     properties: {
+      $schema: {
+        __proto__: null,
+        type: 'string',
+      },
       nodeOptions: {
         __proto__: null,
         additionalProperties: false,