xso: re-implement #[xml(lang)] on top of #[xml(attribute)]

This gives us all the goodies of `default`, `type_` and `codec` without
having to duplicate lots of code (and I think the `match`-iness of the
new macro code is still within limits).

However, we still keep them as separate `#[xml(..)]` attributes, because
their semantics are very different and it is sensible to make them stand
out.

skip-changelog, because `#[xml(lang)]` was introduced in this version.

parsers/src/util/macro_tests.rs

@@ -2467,7 +2467,7 @@ struct Language {
     #[xml(child(default))]
     child: core::option::Option<Box<Language>>,
 
-    #[xml(lang)]
+    #[xml(lang(default))]
     lang: core::option::Option<String>,
 }