Use compilation-mode for rustfmt errors

[roife]

See #204, #395.

This is crucial for user experience, yet it seems to have been neglected for quite some time.

Why can't we reuse rust-compile.el for rustfmt? Are there differences between the diagnostics format from rustfmt and rustc?

[roife]

I’m glad to tackle this issue, but I’m unsure if my approach (reusing rust-compile.el) is correct. I would greatly appreciate it if maintainers could provide some guidance.

[psibi]

Sure, I think it would be good to have it. If you want to take inspiration, it's already available as part of rustic mode: https://github.com/emacs-rustic/rustic/blob/d765680373234a6c231acf20c76b07422afcfdf9/rustic-rustfmt.el#L222

Also, please add tests too. :-)