9138: feat: Implement hover for lints r=Veykril a=Veykril fixes https://github.com/rust-analyzer/rust-analyzer/issues/8857, fixes https://github.com/rust-analyzer/rust-analyzer/issues/3941  We also generate the default lints(and lint groups 🎉) instead now by invoking `rustc -W help` and parsing the output from that. Co-authored-by: Lukas Wirth <lukastw97@gmail.com>