Add ErrSpan extension trait for Result (#12626)

# Description
This adds an extension trait to `Result` that wraps errors in `Spanned`,
saving the effort of calling `.map_err(|err| err.into_spanned(span))`
every time. This will hopefully make it even more likely that someone
will want to use a spanned `io::Error` and make it easier to remove the
impl for `From<io::Error> for ShellError` because that doesn't have span
information.

crates/nu-command/src/filesystem/save.rs

@@ -133,7 +133,7 @@ impl Command for Save {
                                     .spawn(move || stderr.drain()),
                             })
                             .transpose()
-                            .map_err(|e| e.into_spanned(span))?;
+                            .err_span(span)?;
 
                         let res = stream_to_file(stdout, file, span, progress);
                         if let Some(h) = handler {