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/WebAudio/OfflineAudioContext.cpp

@@ -26,7 +26,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<OfflineAudioContext>> OfflineAudioContext::
     // A NotSupportedError exception MUST be thrown if any of the arguments is negative, zero, or outside its nominal range.
     TRY(verify_audio_options_inside_nominal_range(realm, number_of_channels, length, sample_rate));
 
-    return realm.heap().allocate<OfflineAudioContext>(realm, realm, number_of_channels, length, sample_rate);
+    return realm.create<OfflineAudioContext>(realm, number_of_channels, length, sample_rate);
 }
 
 OfflineAudioContext::~OfflineAudioContext() = default;