Create constructs to support the creation of `AWS::CloudFormation::StackSet` resources.

Usage:
```typescript
// the infrastructure to create in target accounts
class AccountAlertTopic extends GuStackForStackSetInstance {
  constructor(id: string, props: GuStackProps) {
    super(id, props);

    new GuSnsTopic(this, "topic-for-alerts");
  }
}

class ParentStack extends GuStackForInfrastructure {
  constructor(scope: App, id: string, props: GuStackProps) {
    super(scope, id, props);

    new GuStackSet(this, `${id}StackSet`, {
      stackSetInstance: new AccountAlertTopic("Alerts", { stack: props.stack }),
      name: "centralised-alarms",
      description: "Provisioning of standard account alerting resources",
      organisationUnitTargets: [ "o-abcde12345" ]
    });
  }
}

// contents of `bin/cdk.ts`
new ParentStack(new App(), "AccountAlarmResources", { stack: "alarms" })
```

This will produce a CloudFormation template like this:
```yaml
Resources:
  AccountAlarmResourcesStackSet:
    Type: AWS::CloudFormation::StackSet
    Properties:
      PermissionModel: SERVICE_MANAGED
      StackSetName: centralised-alarms
      AutoDeployment:
        Enabled: true
        RetainStacksOnAccountRemoval: false
      Description: Provisioning of standard account alerting resources
      Parameters: []
      StackInstancesGroup:
        - DeploymentTargets:
            OrganizationalUnitIds:
              - o-abcde12345
          Regions:
            - Ref: AWS::Region
      Tags:
        - Key: gu:cdk:version
          Value: TEST
        - Key: gu:repo
          Value: guardian/cdk
        - Key: Stack
          Value: alarms
        - Key: Stage
          Value: INFRA
      TemplateBody: |-
        {
          "Resources": {
            "topicforalerts57330FBE": {
              "Type": "AWS::SNS::Topic",
              "Properties": {
                "Tags": [
                  {
                    "Key": "gu:cdk:version",
                    "Value": "TEST"
                  },
                  {
                    "Key": "gu:repo",
                    "Value": "guardian/cdk"
                  },
                  {
                    "Key": "Stack",
                    "Value": "alarms"
                  },
                  {
                    "Key": "Stage",
                    "Value": "INFRA"
                  }
                ]
              }
            }
          }
        }
```

See https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-cloudformation-stackset.html.