Provide #style_name and #responsive_image_style_id in image theme functions

Problem/Motivation

As a front-end developer, I'd expect the ability to customize the fallback <img> tag HTML output inside the <picture> tag would work the same way as with non-responsive images.

I'd expect to be able to add theme suggestions for the image template based on the image style_name used in the fallback and ideally the responsive image style id would also be present.

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEMENAME_theme_suggestions_image_style_alter(array &$suggestions, array $variables) {
  $suggestions[] = $variables['theme_hook_original'] . '__' . $variables['style_name'];
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEMENAME_theme_suggestions_image_alter(array &$suggestions, array $variables) {
  $suggestions[] = $variables['theme_hook_original'] . '__' . $variables['style_name'];
}

However, the responsive_image module does not send the style_name parameter to the image template. And, similarly Drupal core currently doesn't provide a way to pass along the responsive_image_style_id to the various image templates.

Proposed resolution

1. Add responsive_image_style_id as an optional variable sent to all image templates involved in the responsive image render chain. In cases where the image template is not part of a responsive image, the variable would default to NULL.
2. Ensure responsive_image module sends both the style_name and the responsive_image_style_id to the "img_element" render array used as the fallback <img> tag.

Remaining tasks

1. Write patch from initial approach
2. Review patch & approach, confirm that we've covered all possible ways image tags are rendered via responsive_image module
3. Write tests

User interface changes

None.

API changes

A new theme template parameter called responsive_image_style_id will be introduced for the image, image_formatter, and image_style theme functions. The variable will be available to template suggestion hooks, theme preprocess hooks and inside the Twig templates.

Data model changes

None.

Release notes snippet