feat: Support display for WrappedError

[duskmoon314]

This commit supports display attributes for wrapped errors. Users can now customize the display of wrapped errors by providing a string literal or a format string with source as the argument.

Partially solves #7

[duskmoon314]

This PR is more like a POC. It may be better to avoid providing source after the formatting string.

[mcmah309]

Good work! I'm fine with this implementation. It's more explicit and allows for just using a field on source if desired.

In the future we could support

error_set! {
        Y = {
            #[display("Y io error: {}")] // or "Y io error: {0}"
            IoError(std::io::Error),
        };
    }

Without the need to specify source. We'd just need to parse the string if is_str_literal is true, to determine if format! is needed, using something like a try with https://github.com/A1-Triard/dyn-fmt, since format! does not support unused arguments.