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/xpath/eval.rs

@@ -83,6 +83,22 @@ where
     }
 }
 
+impl<T> Evaluatable for Option<T>
+where
+    T: Evaluatable,
+{
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            Some(expr) => expr.evaluate(context),
+            None => Ok(Value::Nodeset(vec![])),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        self.as_ref().is_some_and(|t| T::is_primitive(t))
+    }
+}
+
 impl Evaluatable for Expr {
     fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
         match self {