Add ability to import without using the batch API

Problem/Motivation

It would be nice to do an import procedurally in code without having to use the batch API/drush.

StructureSyncHelper::importCustomBlocks([
    'style' => 'safe',
    'drush' => TRUE, // This will import procedurally, but now work even if not within a drush context.
  ]);

This is mostly useful for using Structure sync in profile installation steps.