Related: pcsg/buero#491

Code stability -> missing return types, qualifier imports, code cleanup, function converts to php8.1

The ErpEntityInterface interface has been expanded to include customer files and the reversal method.
ErpEntityInterface can also be "deleted". However, these cannot be deleted correctly in 90% of cases, hence reversal.

BREAKING CHANGE: ErpEntityInterface extended

New event so that other modules can link into the ERP process as history. + invoice and order creation event added

feat: extends main comments object -> import and sort methods; the main erp process can return the complete history of all erp objects

refactor: started process class -> represented a complete erp process (invoice, transaction, order etc)