Consider moving `miette-derive` to a lighter `syn` alternative

[TheLostLambda]

Apparently syn and its dependencies can often have a significant (negative) impact on compile times — projects like facet are therefore striving to keep their code-bases free of syn.

It might be worth exploring something like unsynn in the future, but I'm not 100% sure if it's ready for that yet? Just something to keep an eye on for now!

Here is how facet is using unsynn: https://github.com/facet-rs/facet/blob/main/facet-derive-parse/src/lib.rs

[TheLostLambda]

Removing syn from the dependency tree does indeed lead to improvements in compilation times — see #435 for what's possible when the derive feature is turned off!