status_messages missing "status_headings" variable

Problem/Motivation

Most (if not all) status_messages templates refer to the "status_headings" variable under normal circumstances.

The core render element itself renders it as such:

[
  '#theme' => 'status_messages',
  '#message_list' => $messages,
  '#status_headings' => [
    'status' => t('Status message'),
    'error' => t('Error message'),
    'warning' => t('Warning message'),
  ],
],

Proposed resolution

Add this to the build array for status messages:

'#status_headings' => [
  'status' => t('Status message'),
  'error' => t('Error message'),
  'warning' => t('Warning message'),
],

User interface changes

There will be front-end rendering changes in a lot of cases.

API changes

None

Data model changes

None