What is the problem you're trying to solve?

I'd like at-rule-no-unknown to respect the atRules property of languageOptions so I can configure my syntax in one place.

Ref: mizdra/css-modules-kit#148

What solution would you like to see?

The rule ignores at-rules defined in languageProperties.

It'd work with the existing ignoreAtRules: [] option rather than replace it.

Please consider contributing if you have time.