CssSyntaxError: not support functions be called at any level

[igauch]

As of Less 3.0, functions can return any kind of Node type, and can be called at any level. but postcss-less not support.
plugin-atrules-feature

• Node Version: v16.14.0
• NPM Version: 8.13.2
• postcss Version: 8.4.14
• postcss-less Version: 6.0.0

If you have a large amount of code to share which demonstrates the problem you're experiencing, please provide a link to your
repository rather than pasting code. Otherwise, please paste relevant short snippets below.

LESS

:root {
  fontSize();
}

JavaScript

    functions.add('fontSize', () => {
      return Object.entries(fontSize).reduce((prev, [key, val]) => {
        return prev + `--font-size-${key}: ${val}px;`
      }, '')
    })

Errors

Unknown word  CssSyntaxError

Expected Behavior

passed

Actual Behavior

failed

How can we reproduce the behavior?

[shellscape]

Happy to review a PR that resolves this.