Member predicate ThreatModelSource::getThreatModel

Gets a string that represents the source kind with respect to threat modeling.

See

• https://github.com/github/codeql/blob/main/docs/codeql/reusables/threat-model-description.rst
• https://github.com/github/codeql/blob/main/shared/threat-models/ext/threat-model-grouping.model.yml