POC to redirect fragments/anchors

Re-roll from latest 1.x-dev/1.8 and for D9 (update renamed 'user.private_tempstore' service).

Improved to use only temp storage and not need to set a 'fragment_redirects' flag in the query string.

I find no branch where the recent three patches apply.

Another reroll of patch in #7, fixing syntax errors.

This patch now depends on the fragment being stored in the hash, and this only works if "Retain query string through redirect." is off. If it is on the fragment is not stored. Se 🐛 Redirect hashes created when not retaining the query string RTBC

This is a good clean solution to redirecting fragments. It is far better than making a separate module. If the fragment reaches the server we should be able to use it. I see it is a major feature request, and have changed it to "Normal".

On the other hand, module maintainers are often reluctant to include additional features. New features may mean more work for them. If it is not included in a published release, a solution needing a large patch on multiple files causes a maintenance headache. I'd like to see some positive reaction from the maintainers before putting more work into it.

This patch is not currently working. The problem is that the install hook simply updates the database, but it doesn't update the schema definition. I have an update update hook that works for my use case, but it's not the correct solution, I'm sure. So we're going to need to correctly handle this.

Here's my custom update hook that creates a backup table, drops the old redirect source column, add the new redirect source column, and then update the new column with the backup table.

/**
 * Add source fragment column.
 */
function redirect_update_8110() {
  $database = \Drupal::database();
  $database->query('CREATE TABLE {redirect_bak} SELECT * FROM {redirect}');
  $field_storage_definition = \Drupal::entityDefinitionUpdateManager()->getFieldStorageDefinition('redirect_source', 'redirect');
  \Drupal::entityDefinitionUpdateManager()->uninstallFieldStorageDefinition($field_storage_definition);
  $field_storage_definition = \Drupal\Core\Field\BaseFieldDefinition::create('redirect_source')
    ->setLabel(t('From'))
    ->setDescription(t("Enter an internal Drupal path or path alias to redirect (e.g. %example1 or %example2). Fragment anchors (e.g. %anchor) are <strong>not</strong> allowed.", ['%example1' => 'node/123', '%example2' => 'taxonomy/term/123', '%anchor' => '#anchor']))
    ->setRequired(TRUE)
    ->setTranslatable(FALSE)
    ->setDisplayOptions('form', [
      'type' => 'redirect_link',
      'weight' => -5,
    ])
    ->setDisplayConfigurable('form', TRUE);
  \Drupal::entityDefinitionUpdateManager()->installFieldStorageDefinition('redirect_source', 'redirect', 'redirect', $field_storage_definition);
  $database->query('UPDATE {redirect} r JOIN {redirect_bak} rb ON r.rid = rb.rid SET r.redirect_source__path = rb.redirect_source__path, r.redirect_source__query = rb.redirect_source__query');
  $database->query('DROP TABLE {redirect_bak}');
}