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

• Install postcss-html
• Pass the default export of that module to customSyntax in a js/ts config file.

https://github.com/ota-meshi/postcss-html/blob/7b8d53529ddc3988c1654c0bf02fe0eee971f6c2/lib/index.js#L19

What minimal configuration is needed to reproduce the bug?

import postcssHtml from "postcss-html"

export default {
  overrides: [
    {files: ["**/*.html"], customSyntax: postcssHtml},
  ],
}

How did you run Stylelint?

npx stylelint

Which Stylelint-related dependencies are you using?

postcss-html

What did you expect to happen?

No error

What actually happened?

Error Custom syntax must be a string or a Syntax object

Do you have a proposal to fix the bug?

Allow function type for customSyntax.