Drupal 8: Add unique ID to theme suggestions for form elements

example.theme.php

<?php

/**
 * @file
 */

/**
 * Implements hook_form_alter()
 */
// Add unique form ID to theme suggestions for form elements.
function example_form_alter(&$form, $form_state, $form_id) {
  $form_type = 'test-type';
  foreach (Element::children($form) as $key) {
    $form[$key]['#form_type'] = $form_type;
  }
  $form['#form_type'] = $form_type;
  $form['actions']['submit']['#form_type'] = $form_type;
  // Modifications to clear/reset button
  if (!empty($form['actions']['reset'])) {
    $form['actions']['reset']['#form_type'] = $form_type;
  }

}

/**
 * Implements hook_theme_suggestions_alter()
 */
function example_theme_suggestions_alter(&$suggestions, $variables, $hook) {
  // Add unique form ID to theme suggestions for form elements.
  $form_hooks = [
    'container',
    'form',
    'select',
    'input',
    'checkboxes',
    'checkbox',
    'form_element',
    'form_element_label',
    'input__checkbox',
    'bef_checkboxes',
    'fieldset',
  ];
  if (in_array($hook, $form_hooks)) {
    _add_unique_form_id_to_suggestions($suggestions, $variables, $hook);
  }
}

/**
 * Helper function that adds a unique form ID to the form element theme suggestions
 */
function _add_unique_form_id_to_suggestions(&$suggestions, $variables, $type) {
  $element = $variables['element'];
  if (isset($element['#form_type'])) {
    $suggestion_suffix = str_replace(['-'], '_', ($type === 'form' ? '' : $element['#id'] . '__' ) . $element['#form_type']);
    $suggestions[] = $type . '__' . $suggestion_suffix;
  }
}