Consider a more substantial shortcuts placeholder

Not a direct result, but this was found while working on 📌 Add performance testing Active .

After checking the MR I realized the resultant code was very similar to the original one we had in Navigation before merging it into core.

There were concerns about this approach expressed in a MR comment that were addressed in the following commit.

Are this concerns still valid and should we think in a different approach for this MR?

So this is what I was worried about in that MR comment:

  '#lazy_builder' => ['navigation.user_lazy_builder:renderNavigationLinks', []],
      '#create_placeholder' => TRUE,
      '#cache' => [
        'keys' => ['user_set_navigation_links'],
        'contexts' => ['user'],
      ],

e.g. the cache context for the placeholder itself was user-specific (I think to show the username). In the shortcuts case, we can always show 'Shortcuts' there, so it's the same for everyone and won't affect cache granularity.

I did have a vague memory of that MR too and kept thinking I was missing something here while working on this, but so far it seems like for the shortcuts link it's always going to be the same for everyone. This might need some before/after manual testing when a user doesn't have access to shortcuts but does have access to navigation, or when their shortcuts are completely empty? But from reading the code if there's an issue with that it might be pre-existing.