Add constants for the local task manager 'levels'

Problem/Motivation

\Drupal\Core\Menu\LocalTaskManagerInterface::getLocalTasks uses 0 and 1 for primary and secondary.
hook_menu_local_tasks_alter discusses placing the altered tasks by level but its not clear in the example (shown below) that the 0 is the level, and not an arbitrary array key.

Steps to reproduce

Review this hook_menu_local_tasks_alter example code and not realise that 0 is signficant.

 // Add a tab linking to node/add to all pages.
  $data['tabs'][0]['node.add_page'] = [
      '#theme' => 'menu_local_task',
      '#link' => [
          'title' => t('Example tab'),
          'url' => Url::fromRoute('node.add_page'),
          'localized_options' => [
              'attributes' => [
                  'title' => t('Add content'),
              ],
          ],
      ],
  ];

Contrast that with

 // Add a tab linking to node/add to all pages.
  $data['tabs'][LocalTaskManagerInterface::LEVEL_PRIMARY]['node.add_page'] = [
      '#theme' => 'menu_local_task',
      '#link' => [
          'title' => t('Example tab'),
          'url' => Url::fromRoute('node.add_page'),
          'localized_options' => [
              'attributes' => [
                  'title' => t('Add content'),
              ],
          ],
      ],
  ];

Proposed resolution

Replace uses of 0 and 1 in local task manager and hook_menu_local_tasks_alter with constants that signify their importance.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet