Add Pointer Events

Close #1585, 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/pointerevents4/",
  "seriesComposition": "full",
  "shortname": "pointerevents4",
  "series": {
    "shortname": "pointerevents",
    "currentSpecification": "pointerevents4",
    "title": "Pointer Events",
    "shortTitle": "Pointer Events",
    "releaseUrl": "https://www.w3.org/TR/pointerevents/",
    "nightlyUrl": "https://w3c.github.io/pointerevents/"
  },
  "seriesVersion": "4",
  "seriesPrevious": "pointerevents3",
  "organization": "W3C",
  "groups": [
    {
      "name": "Pointer Events Working Group",
      "url": "https://www.w3.org/groups/wg/pointer-events/"
    }
  ],
  "release": {
    "url": "https://www.w3.org/TR/pointerevents4/",
    "status": "Working Draft",
    "filename": "Overview.html"
  },
  "nightly": {
    "url": "https://w3c.github.io/pointerevents/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/pointerevents",
    "sourcePath": "index.html",
    "filename": "index.html"
  },
  "title": "Pointer Events",
  "source": "w3c",
  "shortTitle": "Pointer Events",
  "categories": [
    "browser"
  ],
  "standing": "good",
  "tests": {
    "repository": "https://github.com/web-platform-tests/wpt",
    "testPaths": [
      "pointerevents"
    ]
  }
}
```
</details>
<details><summary>Update spec in the same series (1)</summary>

```json
{
  "url": "https://www.w3.org/TR/pointerevents3/",
  "seriesComposition": "full",
  "shortname": "pointerevents3",
  "series": {
    "shortname": "pointerevents",
    "currentSpecification": "pointerevents4",
    "title": "Pointer Events",
    "shortTitle": "Pointer Events",
    "releaseUrl": "https://www.w3.org/TR/pointerevents/",
    "nightlyUrl": "https://w3c.github.io/pointerevents/"
  },
  "seriesVersion": "3",
  "seriesNext": "pointerevents4",
  "organization": "W3C",
  "groups": [
    {
      "name": "Pointer Events Working Group",
      "url": "https://www.w3.org/groups/wg/pointer-events/"
    }
  ],
  "release": {
    "url": "https://www.w3.org/TR/pointerevents3/",
    "status": "Working Draft",
    "filename": "Overview.html"
  },
  "nightly": {
    "url": "https://w3c.github.io/pointerevents/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/pointerevents",
    "sourcePath": "index.html",
    "filename": "index.html"
  },
  "title": "Pointer Events",
  "source": "w3c",
  "shortTitle": "Pointer Events",
  "categories": [
    "browser"
  ],
  "standing": "good",
  "tests": {
    "repository": "https://github.com/web-platform-tests/wpt",
    "testPaths": [
      "pointerevents"
    ]
  }
}
```
</details>

### Tests
These changes look good! 😎

specs.json

@@ -1492,6 +1492,7 @@
     ]
   },
   "https://www.w3.org/TR/pointerevents3/",
+  "https://www.w3.org/TR/pointerevents4/",
   "https://www.w3.org/TR/pointerlock-2/",
   "https://www.w3.org/TR/presentation-api/",
   "https://www.w3.org/TR/proximity/",