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.

.github/labeler.yml

@@ -53,7 +53,7 @@ modifies/internal:
           - ".gitpod.yml"
           - ".markdownlint.yaml"
           - ".spectral.yaml"
-          - ".stylelintrc.yaml"
+          - "stylelint.config.js"
           - ".yamllint.yaml"
           - ".github/**"
           - ".gitea/"

.github/workflows/files-changed.yml

@@ -58,7 +58,7 @@ jobs:
               - "package-lock.json"
               - "Makefile"
               - ".eslintrc.yaml"
-              - ".stylelintrc.yaml"
+              - "stylelint.config.js"
               - ".npmrc"
 
             docs: