Converting paragraphs

#3043559: Implement paragraphs conversion introduced a framwork to convert paragraphs to a different type. The convert action can be enabled as a feature in the stable paragraphs widget settings on the form display page.

Conversions are written as plugin, no conversions exist by default as they depend on the specific paragraph types that are used on a page.

Example:

<?php

namespace Drupal\paragraphs_test\Plugin\paragraphs\Conversion;

use Drupal\paragraphs\ParagraphInterface;
use Drupal\paragraphs\ParagraphsConversionBase;

/**
 * Provides a Paragraphs conversion plugin.
 *
 * @ParagraphsConversion(
 *   id = "paragraphs_test_text_to_text_image",
 *   label = @Translation("Convert to Text Image (No form)"),
 *   source_type = "text",
 *   target_type = {"text_image"}
 * )
 */
class ParagraphsTestTextToTextImage extends ParagraphsConversionBase {

  /**
   * {@inheritdoc}
   */
  public function convert(array $settings, ParagraphInterface $original_paragraph, array $converted_paragraph = NULL) {
    $text = $original_paragraph->get('field_text_demo')->value;
    return [
      [
        'type' => 'text_image',
        'field_text_demo' => [
          'value' => $text,
        ],
      ],
    ];
  }

}

Check the base class for more complex logic, plugins can also provide configuration options.