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/node.rs

@@ -48,7 +48,7 @@ use style::properties::ComputedValues;
 use style::selector_parser::{SelectorImpl, SelectorParser};
 use style::stylesheets::{Stylesheet, UrlExtraData};
 use uuid::Uuid;
-use xml5ever::serialize as xml_serialize;
+use xml5ever::{local_name, serialize as xml_serialize};
 
 use crate::conversions::Convert;
 use crate::document_loader::DocumentLoader;
@@ -1470,6 +1470,21 @@ impl Node {
             .map(|data| data.element_data.borrow().styles.primary().clone())
     }
 
+    /// <https://html.spec.whatwg.org/multipage/#language>
+    pub(crate) fn get_lang(&self) -> Option<String> {
+        self.inclusive_ancestors(ShadowIncluding::Yes)
+            .filter_map(|node| {
+                node.downcast::<Element>().and_then(|el| {
+                    el.get_attribute(&ns!(xml), &local_name!("lang"))
+                        .or_else(|| el.get_attribute(&ns!(), &local_name!("lang")))
+                        .map(|attr| String::from(attr.Value()))
+                })
+                // TODO: Check meta tags for a pragma-set default language
+                // TODO: Check HTTP Content-Language header
+            })
+            .next()
+    }
+
     /// <https://dom.spec.whatwg.org/#assign-slotables-for-a-tree>
     pub(crate) fn assign_slottables_for_a_tree(&self) {
         // NOTE: This method traverses all descendants of the node and is potentially very