More flexible heredoc/nowdoc syntax parsing is impossible on PHP prior to 7.3, causes infinite loop

Problem/Motivation

Localize.Drupal.org went into an infinite loop due to parsing this PHP code:

$form['description'] = [
      '#type' => 'item',
      '#markup' => $this->t('Add links to be displayed in the footer of emails sent by Symfony Mailer. These links will be available in all languages.<br>Example of usage in the template:<br><pre>@code</pre>', [
        '@code' => <<<TWIG
{% if footer_links is not empty %}
  <tr>
    <table border="0" cellpadding="0" cellspacing="0" width="100%">
      <tr>
        {% for link in footer_links %}
          <td>{{ link }}</td>
        {% endfor %}
      </tr>
    </table>
  </tr>
{% endif %}
TWIG,
      ]),
    ];

While this looks fine, the problem is at TWIG, which is "flexible heredoc syntax" shipped in PHP 7.3: https://wiki.php.net/rfc/flexible_heredoc_nowdoc_syntaxes

Previously the heredoc would only be tokenized as heredoc-end if there was a newline, now the comma can be there. Older PHP does not understand the PHP 7.3+ flexible HEREDOC syntax (indented closing markers and tokens on the same line after the heredoc end). Instead of properly tokenizing them, it treats the entire content including the closing marker AND everything that follows as T_ENCAPSED_AND_WHITESPACE, including all subsequent PHP code!

This was triggered by https://www.drupal.org/project/symfony_mailer_addons/releases/1.0.0-beta1.

Proposed resolution

Add graceful fail for this case and test coverage.

Remaining tasks

User interface changes

API changes

Data model changes

LLM disclosure

LLM was used to analyze the problem, excperiment with solutions and write the tests. Everything was manually reviewed.