LibJS+LibWeb: Reduce use of GlobalObject as an intermediary

- Prefer VM::current_realm() over GlobalObject::associated_realm()
- Prefer VM::heap() over GlobalObject::heap()
- Prefer Cell::vm() over Cell::global_object()
- Prefer Wrapper::vm() over Wrapper::global_object()
- Inline Realm::global_object() calls used to access intrinsics as they
  will later perform a direct lookup without going through the global
  object

Userland/Libraries/LibWeb/Fetch/Headers.cpp

@@ -158,8 +158,7 @@ DOM::ExceptionOr<void> Headers::set(String const& name_string, String const& val
 // https://webidl.spec.whatwg.org/#es-iterable, Step 4
 JS::ThrowCompletionOr<void> Headers::for_each(ForEachCallback callback)
 {
-    auto& global_object = wrapper()->global_object();
-    auto& vm = global_object.vm();
+    auto& vm = wrapper()->vm();
 
     // The value pairs to iterate over are the return value of running sort and combine with this’s header list.
     auto value_pairs_to_iterate_over = [&]() -> JS::ThrowCompletionOr<Vector<Fetch::Infrastructure::Header>> {