(Cognito): Ensure a domain is created when VerificationEmailStyle is Link

When a user pool is created with VerificationEmailStyle set to Link, Cognito requires that a domain be present. The CDK could validate this is the case. Info here: https://forums.aws.amazon.com/thread.jspa?threadID=262811

### Use Case

Just to save some headaches down the line.

### Proposed Solution

Throw an exception if a domain doesn't exist.

### Other

* [x] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by locating the Cognito user-pool construct and the handling of VerificationEmailStyle. Confirm how the domain setting is represented, then add the requested exception when Link is selected without a domain. Done means invalid configurations fail during CDK validation with a clear error.

Written by the indexing model from the issue text.