Add a finish event for static HTML generator batch

Setting to NW based on #8.

Making a MR for this and going to address some of the points.

> we also generate static HTML via the command line and tome_static_cron,

This to me really suggests that more of the static generation code should be in an API and less in the running/UI code -- Drush/cron/batch. But that's for another issue.

I can add the success event to the Drush StaticCommand class, but I'm not sure where the failure event would get called? There's this in exportPaths():

    if (!empty($collected_errors)) {
      $this->displayErrors($collected_errors);
    }

but exportPaths is called recursively in StaticCommand AND by the inner command StaticExportPathCommand -- so the event would be fired multiple times which we don't want. (See 📌 refactor code to remove Drush command calling another command Active .)

For cron it's even worse -- I'm not suere where we'd call the success event, as things are put into a queue. I guess we could add a final queue item whose sole purpose is to fire the event?

I think the success/failure distinction needs more thought.

This code in the StaticGeneratorForm (in the MR version) shows this:

    if (!$success) {
      # This is a failure of the whole batch with Batch API.
      $this->messenger()->addError($this->t('Static build failed - consult the error log for more details.'));
      $event = new BuildFinishEvent(FALSE);
      $this->eventDispatcher->dispatch($event, TomeStaticEvents::BUILD_FINISH);
      return;
    }
    if (!empty($results['errors'])) {
      # This is exceptions from making requests with StaticGenerator.
      # This counts as a success!
      foreach ($results['errors'] as $error) {
        $this->messenger()->addError($error);
      }
    }

We have batch API failure, but also a case where we have error messages from StaticGenerator - but that's treated as a success.

And furthermore, error messages from StaticGenerator do NOT include errors if requests for site paths produce a 500 from the site -- those errors are just ignored -- see ✨ StaticGenerator::requestPath() doesn't do anything with a request that produces a 500 error Needs review .

I've updated the patch so it applies again. Also triggered the event after executing the tome:static command.