[ticket/12561] Add "after" check to schema_generator for columns_add

It is now possible to use before and after definitions in migrations when using add_columns. However, this isn't processed (yet) by create_schema_files.php, which will result in it being output directly to the schema.json, instead of being used for actual location placement.

https://tracker.phpbb.com/browse/PHPBB3-12561
PHPBB3-12561