Allow callers to verify support for purl_type

What does this MR do?

This MR adds the SemverDialects.package_registry_supported? public function in order to give callers a way to verify whether a particular package registry type is supported by SemverDialects.

What are the relevant issue numbers?

This is related to a discussion on a separate MR about how to decouple package registry support in the monolith from the gem: gitlab-org/gitlab!157358

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for this project, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer