LibJS: Always allocate ExecutionContext objects on the malloc heap

Instead of allocating these in a mixture of ways, we now always put
them on the malloc heap, and keep an intrusive linked list of them
that we can iterate for GC marking purposes.

Userland/Libraries/LibJS/Runtime/Realm.cpp

@@ -42,7 +42,7 @@ ThrowCompletionOr<NonnullOwnPtr<ExecutionContext>> Realm::initialize_host_define
     auto realm = MUST_OR_THROW_OOM(Realm::create(vm));
 
     // 2. Let newContext be a new execution context.
-    auto new_context = make<ExecutionContext>(vm.heap());
+    auto new_context = ExecutionContext::create(vm.heap());
 
     // 3. Set the Function of newContext to null.
     new_context->function = nullptr;