Feature Request: Extract theme creation from SubthemeManager::submitForm

Implemented the change I mentioned. Please check if this is good enough for production. Thanks.

If things go right, I should be able to create a new sub theme by running this in bash with drush:

drush php:eval "
$m = new \Drupal\bootstrap5\SubthemeManager(\Drupal::getContainer()->get('file_system'), \Drupal::getContainer()->get('messenger'));
echo $m->createSubtheme('mysubtheme', 'My Bootstrap 5 Subtheme', 'themes/custom');
"