The initial-value descriptor in an @property rule takes any value. (

#674)

<!-- 🙌 Thanks for contributing! Adding details below will help us to
merge your PR faster. -->

### Description

The `initial-value` descriptor of `@property` rules is not limited to
strings.
It permits any value.

### Motivation

`<string>` is incorrect.

### Additional details

see :
https://drafts.css-houdini.org/css-properties-values-api/#the-syntax-descriptor

### Related issues and pull requests

N/A

<!-- 🔨 If this fully resolves a GitHub issue, use "Fixes #123" -->
<!-- 👉 Highlight related pull requests using "Relates to #123" -->
<!-- ❗ If another pull request should be merged first, use "**Depends
on:** #123" -->

---------

Co-authored-by: Onkar Ruikar <87750369+OnkarRuikar@users.noreply.github.com>

css/at-rules.json

@@ -455,7 +455,7 @@
         "status": "experimental"
       },
       "initial-value": {
-        "syntax": "<string>",
+        "syntax": "<declaration-value>?",
         "media": "all",
         "initial": "n/a (required)",
         "percentages": "no",