src/language-css/utils.js/insideValueFunctionNode
function insideValueFunctionNode(path, functionName) {
  const funcAncestorNode = getAncestorNode(path, "value-func");
  return (
    funcAncestorNode &&
    funcAncestorNode.value &&
    funcAncestorNode.value.toLowerCase() === functionName
  );
}