LibWeb/DOM: Work around GCC 14 warning on always true `is<T>()`

GCC 14 emits a warning when an always succeeding `dynamic_cast`'s return
value is compared to NULL inside the `AK::is<T>(U)` template when `T` ==
`U`.

While warning on tautological `is` calls seems useful, it's a bit
awkward when it comes from a function template where the cast may fail
in some instantiation. There is a GCC bug open for it:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115664

Work around the warning by performing the algorithm on the base type
(`EventTarget`), with a wrapper that casts it to the more specialized
input type.

Userland/Libraries/LibWeb/DOM/Utils.h

@@ -13,8 +13,7 @@
 namespace Web::DOM {
 
 // https://dom.spec.whatwg.org/#retarget
-template<typename T>
-T* retarget(T* a, T* b)
+inline EventTarget* retarget_impl(EventTarget* a, EventTarget* b)
 {
     // To retarget an object A against an object B, repeat these steps until they return an object:
     for (;;) {
@@ -39,4 +38,11 @@ T* retarget(T* a, T* b)
     }
 }
 
+// https://dom.spec.whatwg.org/#retarget
+template<typename T>
+T* retarget(T* a, T* b)
+{
+    return static_cast<T*>(retarget_impl(a, b));
+}
+
 }