LibJS+LibWeb: Use realm.create<T> instead of heap.allocate<T>

The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.

As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.

Libraries/LibWeb/HTML/ImageRequest.cpp

@@ -25,7 +25,7 @@ JS_DEFINE_ALLOCATOR(ImageRequest);
 
 JS::NonnullGCPtr<ImageRequest> ImageRequest::create(JS::Realm& realm, JS::NonnullGCPtr<Page> page)
 {
-    return realm.heap().allocate<ImageRequest>(realm, page);
+    return realm.create<ImageRequest>(page);
 }
 
 ImageRequest::ImageRequest(JS::NonnullGCPtr<Page> page)