fix: Check Type instance has DscResource attribute

[haodeon]

PR Summary

PR #797 introduced a workaround to detect if DSC resources were ClassBased. This PR adds an additional validation by checking the instance has the DscResource attribute.

PR Context

The workaround incorrectly sets ImplementationDetail to ClassBased when there is another type with the same name, like TimeZone or Environment.

[Gijsreyn]

Nice catch @haodeon!