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-cmd-plugin/src/commands/plugin/add.rs

@@ -106,9 +106,7 @@ apparent the next time `nu` is next launched with that plugin cache file.
 
         let shell_expanded = shell
             .as_ref()
-            .map(|s| {
-                nu_path::canonicalize_with(&s.item, &cwd).map_err(|err| err.into_spanned(s.span))
-            })
+            .map(|s| nu_path::canonicalize_with(&s.item, &cwd).err_span(s.span))
             .transpose()?;
 
         // Parse the plugin filename so it can be used to spawn the plugin