XPath: implement lang() and id() core functions (#34594)

XPath's `lang()` and `id()` functions were still unimplemented.

Also:
* Add WPT tests for `id()`.
* Fix uniqueness check in `NodesetHelpers::document_order_unique`.
* Tweak the AST a bit to make it clearer to express "no predicates".
* Fix a parsing bug where "/" was attempted before "//", leaving the
"//" branch as always unused.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #34593 
- [x] There are tests for these changes

---------

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

components/script/dom/xpathresult.rs

@@ -54,7 +54,7 @@ impl TryFrom<u16> for XPathResultType {
     }
 }
 
-#[derive(JSTraceable, MallocSizeOf)]
+#[derive(Debug, JSTraceable, MallocSizeOf)]
 pub(crate) enum XPathResultValue {
     Boolean(bool),
     /// A IEEE-754 double-precision floating point number