Update parsing logic for transaction details to support longer GVC and new separator

[gregorybrzeski]

This update modifies the handling of transaction details parsing to accommodate the following use case:

• GVC length with four digits
• Use of < as a separator

Additionally, I wanted to raise the idea of offering more flexibility by allowing users to configure these parameters (GVC length and separator) via environment variables. Happy to provide a PR to handle that.

Please let me know your thoughts on this approach, and if you'd be open to merging this change.

Looking forward to your feedback.

Example of my use case MT940 transaction message:

:61:1901310131DN30,00NCHGNONREF//10958
Opłaty okresowe
:86:8090<00Pobranie opłaty<1010958
<20Oplaty i prowizje
<63REF452015339260_19968

[gregorybrzeski]

Hi! I appreciate your time reviewing this PR. If there are any additional changes needed, I'm happy to make them.