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/Document.cpp

@@ -3310,6 +3310,8 @@ void Document::run_the_update_intersection_observations_steps(HighResolutionTime
 // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#start-intersection-observing-a-lazy-loading-element
 void Document::start_intersection_observing_a_lazy_loading_element(Element& element)
 {
+    VERIFY(element.is_lazy_loading());
+
     auto& realm = this->realm();
 
     // 1. Let doc be element's node document.
@@ -3333,7 +3335,8 @@ void Document::start_intersection_observing_a_lazy_loading_element(Element& elem
                 // 2. If entry.isIntersecting is true, then set resumptionSteps to entry.target's lazy load resumption steps.
                 if (entry.is_intersecting()) {
                     // 5. Set entry.target's lazy load resumption steps to null.
-                    resumption_steps = verify_cast<HTML::HTMLImageElement>(*entry.target()).take_lazy_load_resumption_steps({});
+                    VERIFY(entry.target()->is_lazy_loading());
+                    resumption_steps = entry.target()->take_lazy_load_resumption_steps({});
                 }
 
                 // 3. If resumptionSteps is null, then return.