Add Verifiable Credentials JSON Schema Specification (#2054)

Close #2049, adding the suggested spec to the list.

### Changes to `index.json`
This update would trigger the following changes in `index.json`:

<details><summary>Add spec (1)</summary>

```json
{
  "url": "https://www.w3.org/TR/vc-json-schema/",
  "seriesComposition": "full",
  "shortname": "vc-json-schema",
  "series": {
    "shortname": "vc-json-schema",
    "currentSpecification": "vc-json-schema",
    "title": "Verifiable Credentials JSON Schema Specification",
    "shortTitle": "Verifiable Credentials JSON Schema",
    "releaseUrl": "https://www.w3.org/TR/vc-json-schema/",
    "nightlyUrl": "https://w3c.github.io/vc-json-schema/"
  },
  "organization": "W3C",
  "groups": [
    {
      "name": "Verifiable Credentials Working Group",
      "url": "https://www.w3.org/groups/wg/vc/"
    }
  ],
  "release": {
    "url": "https://www.w3.org/TR/vc-json-schema/",
    "status": "Candidate Recommendation Draft",
    "filename": "Overview.html"
  },
  "nightly": {
    "url": "https://w3c.github.io/vc-json-schema/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/vc-json-schema",
    "sourcePath": "index.html",
    "filename": "index.html"
  },
  "title": "Verifiable Credentials JSON Schema Specification",
  "source": "w3c",
  "shortTitle": "Verifiable Credentials JSON Schema",
  "categories": [],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

Co-authored-by: browser-specs-bot <>

Author: browser-specs-bot

specs.json

@@ -1767,6 +1767,7 @@
   "https://www.w3.org/TR/vc-di-ecdsa/",
   "https://www.w3.org/TR/vc-di-eddsa/",
   "https://www.w3.org/TR/vc-jose-cose/",
+  "https://www.w3.org/TR/vc-json-schema/",
   "https://www.w3.org/TR/vibration/",
   "https://www.w3.org/TR/virtual-keyboard/",
   {