LibJS: Make PromiseCapability GC-allocated

A struct with three raw pointers to other GC'd types is a pretty big
liability, let's just turn this into a Cell itself.
This comes with the additional benefit of being able to capture it in
a lambda effortlessly, without having to create handles for individual
members.

Userland/Libraries/LibJS/Runtime/AsyncGenerator.cpp

@@ -21,9 +21,7 @@ void AsyncGenerator::visit_edges(Cell::Visitor& visitor)
     for (auto const& request : m_async_generator_queue) {
         if (request.completion.value().has_value())
             visitor.visit(*request.completion.value());
-        visitor.visit(request.capability.promise);
-        visitor.visit(request.capability.reject);
-        visitor.visit(request.capability.resolve);
+        visitor.visit(request.capability);
     }
 }