LibWeb: Extract shared lazy-loading behavior into a base class

`<iframe>` and `<img>` tags share the same spec for several aspects of
lazy-loading: how the `loading` attribute works, the "will lazy load
element" steps, and a member for storing the lazy-load resumption
steps. So let's share the implementation by using a base class.

This mostly involves moving things around. However, we also change the
`start_intersection_observing_a_lazy_loading_element()` method to take
a LazyLoadingElement, and operate on one, instead of always casting to
HTMLImageElement.

We do unfortunately have to do some shenanigans to make the cast work,
by adding a virtual function stub in DOM::Element.

Userland/Libraries/LibWeb/DOM/Element.h

@@ -20,6 +20,7 @@
 #include <LibWeb/DOM/Slottable.h>
 #include <LibWeb/HTML/AttributeNames.h>
 #include <LibWeb/HTML/EventLoop/Task.h>
+#include <LibWeb/HTML/LazyLoadingElement.h>
 #include <LibWeb/HTML/ScrollOptions.h>
 #include <LibWeb/HTML/TagNames.h>
 #include <LibWeb/IntersectionObserver/IntersectionObserver.h>
@@ -372,6 +373,11 @@ public:
 
     Optional<FlyString> const& id() const { return m_id; }
 
+    virtual JS::GCPtr<JS::HeapFunction<void()>> take_lazy_load_resumption_steps(Badge<DOM::Document>)
+    {
+        return nullptr;
+    }
+
 protected:
     Element(Document&, DOM::QualifiedName);
     virtual void initialize(JS::Realm&) override;