Returns a settings form to configure this CKEditor plugin.
If the plugin's behavior depends on extensive options and/or external data, then the implementing module can choose to provide a separate, global configuration page rather than per-text-editor settings. In that case, this form should provide a link to the separate settings page.
array $form: An empty form array to be populated with a configuration form, if any.
array $form_state: The state of the entire filter administration form.
\Drupal\editor\Plugin\Core\Entity\Editor $editor: A configured text editor object.
array|FALSE A render array for the settings form, or FALSE if there is none.
function settingsForm(array $form, array &$form_state, Editor $editor);