Reevaluate decision to not add support for nightly diagnostics API

[ModProg]

While the diagnostics are still nightly, they are able to produce significantly better error messages, it could make sense to support them both to prepare for eventual stabilization and to improve the usage in nightly crates.

[JustusFluegel]

It would be possible to enable them using conditional compilation if manyhow is compiled on a nightly compiler.

[ModProg]

It would be possible to enable them using conditional compilation if manyhow is compiled on a nightly compiler.

yeah that would be the idea, and at some point when they stabelize we could remove the generation of the compile_error!{} completly.