AWS::CloudFormation::StackSet does not support AccountFilterType of UNION

The example below does not work due to a bug in cloudformation stack sets. I have opened an AWS case to address the issue but also creating this for awareness.

Caused by: https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-cloudformation/issues/100

```
additionalAccounts: [
'1234567'
],
organizationalUnits: [
ou-12345
],
parameterOverrides: {},
}),
```

The issue names no repository file, test, or entry point. Start by reproducing the TypeScript StackSet example and reading the linked AWS CloudFormation provider issue; completion would require a confirmed resolution for UNION support rather than an explicitly scoped repository change.

Written by the indexing model from the issue text.