[Feature] Concurrency for async registrations

[naamapps]

It would be nice if the package knew to generate async registrations in concurrent fashion.

For example: A service and B service are async singletons and they don't depend on any other service. With the current version the package will generate this code (preResolve is true).

  await gh.singletonAsync<_i11.A>(
    () => A.create(),
    preResolve: true,
  );
  await gh.singletonAsync<_i12.B>(
    () => B.create(),
    preResolve: true,
  );

If concurrency will be implemented the code will become like so:

  await Future.wait([
    gh.singletonAsync<_i11.A>(
    () => A.create(),
       preResolve: true,
    ),
    gh.singletonAsync<_i12.B>(
    () => B.create(),
       preResolve: true,
    )
  ]);

Why we need this feature? This will make the app start up faster! Now my app starts in about 1 second, which I think can be improved drastically if the GetIt setup was using concurrency.

Thanks.