Add SHACL 1.2 Core (#1743)

Close #1739, 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/shacl12-core/",
  "seriesComposition": "full",
  "shortname": "shacl12-core",
  "series": {
    "shortname": "shacl-core",
    "currentSpecification": "shacl12-core",
    "title": "SHACL Core",
    "shortTitle": "SHACL Core",
    "releaseUrl": "https://www.w3.org/TR/shacl-core/",
    "nightlyUrl": "https://w3c.github.io/data-shapes/shacl-core/"
  },
  "seriesVersion": "1.2",
  "categories": [],
  "organization": "W3C",
  "groups": [
    {
      "name": "Data Shapes Working Group",
      "url": "https://www.w3.org/groups/wg/data-shapes/"
    }
  ],
  "release": {
    "url": "https://www.w3.org/TR/shacl12-core/",
    "status": "First Public Working Draft",
    "filename": "Overview.html"
  },
  "nightly": {
    "url": "https://w3c.github.io/data-shapes/shacl12-core/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/data-shapes",
    "sourcePath": "shacl12-core/index.html",
    "filename": "index.html"
  },
  "title": "SHACL 1.2 Core",
  "source": "w3c",
  "shortTitle": "SHACL 1.2 Core",
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

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

Author: browser-specs-bot

specs.json

@@ -1618,6 +1618,12 @@
   "https://www.w3.org/TR/selectors-nonelement-1/",
   "https://www.w3.org/TR/server-timing/",
   "https://www.w3.org/TR/service-workers/",
+  {
+    "categories": [
+      "-browser"
+    ],
+    "url": "https://www.w3.org/TR/shacl12-core/"
+  },
   {
     "url": "https://www.w3.org/TR/sparql12-entailment/",
     "categories": [