Add Time Zone Canonicalization proposal

Close #1460, 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-canonical-tz/",
  "seriesComposition": "full",
  "shortname": "tc39-canonical-tz",
  "series": {
    "shortname": "tc39-canonical-tz",
    "currentSpecification": "tc39-canonical-tz",
    "title": "Time Zone Canonicalization proposal",
    "shortTitle": "Time Zone Canonicalization",
    "nightlyUrl": "https://tc39.es/proposal-canonical-tz/"
  },
  "organization": "Ecma International",
  "groups": [
    {
      "name": "TC39",
      "url": "https://tc39.es/"
    }
  ],
  "nightly": {
    "url": "https://tc39.es/proposal-canonical-tz/",
    "status": "Editor's Draft",
    "alternateUrls": [],
    "repository": "https://github.com/tc39/proposal-canonical-tz",
    "sourcePath": "spec.html",
    "filename": "index.html"
  },
  "title": "Time Zone Canonicalization proposal",
  "source": "spec",
  "shortTitle": "Time Zone Canonicalization",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

specs.json

@@ -366,6 +366,7 @@
       "ecmascript"
     ]
   },
+  "https://tc39.es/proposal-canonical-tz/",
   {
     "url": "https://tc39.es/proposal-change-array-by-copy/",
     "standing": "discontinued",