feat(css): Update syntax for @page/size descriptor (#977)

Author: skyclouds2001

css/at-rules.json

@@ -444,7 +444,7 @@
         "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@page/page-orientation"
       },
       "size": {
-        "syntax": "<length>{1,2} | auto | [ <page-size> || [ portrait | landscape ] ]",
+        "syntax": "<length [0,∞]>{1,2} | auto | [ <page-size> || [ portrait | landscape ] ]",
         "media": [
           "visual",
           "paged"