feat: Emit a warning if a type is marked as `NeverClone` but it implements the `Copy` trait

[LukeMathWalker]

This is a good first issue for those who want to start working on pavexc, the compiler side of Pavex.

If you're interested in tackling it, comment here to claim it and I'll be happy to provide guidance.

[donmai-me]

Hi! I just submitted a pr that hopefully implements this feature. I just mostly copied code from clonables_can_be_cloned function. Any feedback is welcome thanks!