Bump major version of CSS package to v5.0.0

Breaking change introduced by the switch to Reffy v9.0.0: at-rules are now
reported under a new `atrules` property (that itself is not a breaking change),
and descriptors are now to be found under the at-rule they apply to (so no more
`descriptors` properties at the root level).

Also note that all values that appear in the CSS package can now be parsed with
css-tree, meaning that there are no more exceptions.

packages/css/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@webref/css",
   "description": "CSS definitions of the web platform",
-  "version": "4.4.0",
+  "version": "5.0.0",
   "repository": {
     "type": "git",
     "url": "https://github.com/w3c/webref.git"