Highlight unresolved derives as `unresolvedReference`

[Veykril]

While we can't highlight tokens in random attributes as unresolvedReference due to them potentially not being reused in the macro expansion, we can do this for derive attributes specifically as it is a special builtin attribute anyways.

[TheDoctor314]

I would like to work on this but I have no idea where to begin. Could you point me in the right direction?

[Veykril]

The place for this starts here https://github.com/rust-analyzer/rust-analyzer/blob/9c4e9e46179b638301a2a79b635295f6a36736b1/crates/hir_def/src/nameres/collector.rs#L1355

Then you can just orientate yourself at how DefDiagnostics are handled, it's an enum you'll add a variant to as well from there on it should be fairly similar to the other variants I believe.