Implement config schema for block.settings.menu_block.*.{follow, follow_parent}

Attached patch implements configuration schema data for block.settings.menu_block.*.{follow, follow_parent} as well as an hook_update_N() to fix existing block configuration.

We can take inspiration from an issue in Drupal core that added a key 'expand_all_items' to system_menu_block configuration #2594425: Add the option in system menu block to "Expand all items in this tree"

1. Move the update from hook_update_N() to a post_update function, so the full Drupal API becomes available
2. Use the ConfigEntityUpdater to make updating easier.

The relevant code in the related issue:

/**
 * Initialize 'expand_all_items' values to system_menu_block.
 */
function system_post_update_add_expand_all_items_key_in_system_menu_block(&$sandbox = NULL) {
  if (!\Drupal::moduleHandler()->moduleExists('block')) {
    return;
  }
  \Drupal::classResolver(ConfigEntityUpdater::class)->update($sandbox, 'block', function ($block) {
    return strpos($block->getPluginId(), 'system_menu_block:') === 0;
  });
}

The post_update hook works great! Two minor points before this is ready:

1. Menu block does not list Block as a dependency, so we'll have to check if the block module is enabled similar to the example code in #9. Otherwise the update will crash your site:
   

   The following updates are pending:
   
   menu_block module :
     Implement config schema for the menu_block settings follow.
   
   Do you wish to run all pending updates? (y/n): y
   Post updating menu_block                                                                                                                                                  [ok]
   Failed: Drupal\Component\Plugin\Exception\PluginNotFoundException: The "block" entity type does not exist. in                                                   [error]
   Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 150 of /[...]/core/lib/Drupal/Core/Entity/EntityTypeManager.php).
   Cache rebuild complete.                                                                                                                                                   [ok]
   Finished performing updates.
   

2. +++ b/menu_block.post_update.php
   @@ -0,0 +1,26 @@
   +      return FALSE;
   +    })
   +  ;
   

   Code style: the ; should be on the previous line.

The post_update implementation now works correctly when the Block module is not installed. It also updates the block configuration succesfully to the new configuration schema.

The indentation in #12 is correct according to the Drupal Code sniffer.