Add support for splitsing a text block in 2 or 3 columns

The access can be handled in a block_type_form_alter

e.g. a variation of the following (this was only to exclude it from the 2 column layout)

function MODULE_block_type_form_alter(array &$form, FormStateInterface &$form_state, string $block_type) {

  switch ($block_type) {
    case 'cb_text':
      $current_section = $form_state->getFormObject()->getCurrentSection();
      if ($current_section->getLayoutId() === 'rs_two_col') {
        $form['field_columns']['#access'] = FALSE;
      }
      break;

  }
}

And the class can be added as following

function MODULE_block_content_view_alter(array &$build, BlockContentInterface $entity, EntityViewDisplayInterface $display) {
  if ($entity->hasField('field_columns')) {
    $columns = $entity->get('field_columns')->value ?? 'one-column';
    $build['#attributes']['class'][] = 'block--' . $columns;
  }
}