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/LibJS/Bytecode/Interpreter.cpp

@@ -1742,7 +1742,7 @@ inline ThrowCompletionOr<Object*> get_object_property_iterator(VM& vm, Value val
             }
         },
         1, vm.names.next);
-    return vm.heap().allocate<IteratorRecord>(realm, realm, object, callback, false).ptr();
+    return realm.create<IteratorRecord>(realm, object, callback, false).ptr();
 }
 
 ByteString Instruction::to_byte_string(Bytecode::Executable const& executable) const