Add `languageOptions.syntax` support to `function-no-unknown`

We can use csstree to check for unknown functions, as we did for property-no-unknown in:

• https://github.com/stylelint/stylelint/pull/9099

It'll remove the css-functions-list dep:

https://github.com/stylelint/stylelint/blob/5da599ed68262c1df00d9bbe190abff28fec6243/package.json#L137

And add support for languageOptions.syntax to the rule.

(May require a slight change in prefixed behaviour, so adding to 18.)