In this commit, several changes have been made to streamline the Manufacturer and Process classes
within the ERP module:

- Replaced the non-empty check for 'firstname' and 'lastname' attributes in the User class with an
`empty()` function, resulting in cleaner code without changing the functionality.
- Replaced the argument in `User->activate()` from `false` to `''` for consistency.
- Removed unnecessary annotations and some unused `protected` properties from the Process class.
- Altered the procedure to retrieve `salesOrder` from the Entity in the Process class. Now, the
method first tries to retrieve it via `getPaymentDataEntry()`, and if unavailable, it attempts to
fetch it using `getCustomDataEntry()`. This ensures a more robust and reliable retrieval of the
'salesOrder' property.
- Added `@phpstan-ignore-next-line` in the Processes class to prevent phpstan from throwing
unwarranted warnings regarding the following code line in specific scenarios.

Related: #103

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)