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.
(cherry picked from commit f31a88d3cb64106e75bbe8a3502856db71dbacfc)
Conflicts:
- .github/labeler.yml
- .github/workflows/files-changed.yml
Removed both, Gitea specific files.
