Feature Request: Support primary_span attr

[bvanjoi]

Currently, we support the label attribute to display the error source span. However, it seems that an error struct can have multiple labels, which sometimes results in the absence of a primary label. Therefore, I recommend adding a primary_span to indicate the primary span of the diagnostic, ensuring that the main message is clearly associated with a specific location.

The usage should be as follows:

#[derive(Error, Debug, Diag)]
#[error(message)]
struct MyError {
    #[primary_span]
    span: Span
}

[zkat]

I wrote about this a couple of years ago but never really made a separate issue for it: #125 (comment)

My main thought about this proposal is I'd rather reuse #[span] and just give it an optional argument to specify whether something is primary. Other than that, I think something like this is long overdue.