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>

tests/wpt/meta/domxpath/fn-lang.html.ini

@@ -1,21 +0,0 @@
-[fn-lang.html]
-  [lang("en"): <root><match lang="en"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><match lang="EN"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><match lang="en-us"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><unmatch></unmatch></root>]
-    expected: FAIL
-
-  [lang("ja"): <root lang="ja"><match></match></root>]
-    expected: FAIL
-
-  [lang("ja"): <root lang="ja-jp"><unmatch lang="ja_JP"></unmatch></root>]
-    expected: FAIL
-
-  [lang("ko"): <root><unmatch lang="Ko"></unmatch></root>]
-    expected: FAIL

tests/wpt/meta/domxpath/node-sets.html.ini

@@ -1,3 +0,0 @@
-[node-sets.html]
-  [| operator should evaluate both sides of expressions with the same context node]
-    expected: FAIL