Add stylelint-value-no-unknown-custom-properties and convert stylelint config to js (#30117)

Add
[`stylelint-value-no-unknown-custom-properties`](https://github.com/csstools/stylelint-value-no-unknown-custom-properties)
which lints for undefined CSS variables. No current violations.

To make it work properly with editor integrations, I had to convert the
config to JS to be able to pass absolute paths to the plugin, but this
is a needed change anyways.

package.json

@@ -90,6 +90,7 @@
     "stylelint": "16.3.0",
     "stylelint-declaration-block-no-ignored-properties": "2.8.0",
     "stylelint-declaration-strict-value": "1.10.4",
+    "stylelint-value-no-unknown-custom-properties": "6.0.1",
     "svgo": "3.2.0",
     "updates": "16.0.0",
     "vite-string-plugin": "1.1.5",