feat: Add support for defmt

[darkwater]

Fixes #436

[darkwater]

Hmm, CI failed on Windows. Defmt is mostly intended for embedded, and I'm not sure what the non-embedded story is currently.

The actual error happens because there's no defmt backend, I think it doesn't error anywhere else because the symbols are correctly removed because they're unused.

I think the best way is to just #[cfg(not(target_os = "windows"))] because defmt currently doesn't support non-ELF binaries anyway.

[ParkMyCar]

Hey @darkwater, thanks for the PR! Apologies for the delay in review but this looks good to me. I'm going to wait to do a release until #439 merges, and then together these will go out with v0.10