What minimal example or steps are needed to reproduce the bug?

@function --foo() {}

What minimal configuration is needed to reproduce the bug?

{
  "rules": {
    "at-rule-no-unknown": true
  }
}

How did you run Stylelint?

demo

Which Stylelint-related dependencies are you using?

{
  "stylelint": "latest"
}

Resources

support : https://caniuse.com/wf-function
spec: https://drafts.csswg.org/css-mixins-1/#function-rule
related: #1612

What actually happened?

Unexpected unknown at-rule "@function" (at-rule-no-unknown)

Do you have a proposal to fix the bug?

update https://github.com/stylelint/stylelint/blob/main/lib/reference/atKeywords.mjs