Add lint for publically constructable Unsafe types

[GnomedDev]

This is a pretty dangerous mistake that I feel like is really easy to run into with this pattern, and someone did recently on the Rust community discord (although with Unprotected instead of Unsafe), so let's get this linted against.

changelog: [constructable_unsafe_type]: Add lint for publically constructable Unsafe types

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[GnomedDev]

I just got the suggestion to change this from linting based on type name to linting if the only function constructors are unsafe, which sounds like a better idea.