Add Error.isError

Close #1590, 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://tc39.es/proposal-is-error/",
  "seriesComposition": "full",
  "shortname": "tc39-is-error",
  "series": {
    "shortname": "tc39-is-error",
    "currentSpecification": "tc39-is-error",
    "title": "Error.isError",
    "shortTitle": "Error.isError",
    "nightlyUrl": "https://tc39.es/proposal-is-error/"
  },
  "organization": "Ecma International",
  "groups": [
    {
      "name": "TC39",
      "url": "https://tc39.es/"
    }
  ],
  "nightly": {
    "url": "https://tc39.es/proposal-is-error/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/tc39/proposal-is-error",
    "sourcePath": "spec.emu",
    "filename": "index.html"
  },
  "title": "Error.isError",
  "source": "spec",
  "shortTitle": "Error.isError",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

specs.json

@@ -452,6 +452,7 @@
       "ecma-402"
     ]
   },
+  "https://tc39.es/proposal-is-error/",
   {
     "url": "https://tc39.es/proposal-is-usv-string/",
     "standing": "discontinued",