Add block type to description

This makes sense as an addition.

However rather than looking into the $form for the admin_label we should be able to load the block with something like $block = $form_state->getFormObject()->getEntity() and then use $block->getPlugin()->getPluginDefinition()['admin_label'] instead. Would be nice to add a quick test for this as well.

$form_state->getFormObject() is an instance of Drupal\layout_builder\Form\AddBlockForm. There is no getEntity() available there, and the $block instance variable is protected so we can't get at it from this class.