@Override
    public <T> T accept(ExpressionVisitor<T> visitor) {
        return visitor.visitFlatten(this);
    }
Flattens the wrapped expression into an array.

@see <a href="https://jmespath.org/specification.html#flatten-operator">Flatten Operator</a>
smithy-jmespath/src/main/java/software/amazon/smithy/jmespath/ast/FlattenExpression.java