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.
silverwind