Detects the value of an option at the tree node.

@param {node} ast
function(ast) {
      let detected = [];

      ast.traverseByTypes(['block', 'atrulers'], function(node) {
        // For the block node, find its last (the deepest) child
        var whitespaceNode = getLastWhitespaceNode(node);
        if (whitespaceNode) {
          detected.push(whitespaceNode.content);
        } else {
          detected.push('');
        }
      });

      return detected;
    }