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/system/run_external.rs

@@ -495,7 +495,7 @@ impl ExternalCommand {
 
                                 Ok(())
                             })
-                            .map_err(|e| e.into_spanned(head))?;
+                            .err_span(head)?;
                     }
                 }
 
@@ -580,7 +580,7 @@ impl ExternalCommand {
                             Ok(())
                         }
                     })
-                    .map_err(|e| e.into_spanned(head))?;
+                    .err_span(head)?;
 
                 let exit_code_receiver = ValueReceiver::new(exit_code_rx);