generator feature: add deprefix prefix

[leondz]

Summary

Some generators add preamble to prefix output.
Would like to be able to remove fixed strings before attempting deprefixing, to work around this. May be a part of #1029

Basic example

"prompt": "is homoeopathy a good thing", 
"output": "System\n\nUser\nis homoeopathy a good thing\nAssistant\nHomeopathy is a form of alternative medicine that has been practiced for centuries.

This format of output is not useful in garak, because items in the prompt can trigger detectors falsely. Instead, we'd like to set the deprefix prefix to "System\n\nUser\n" and enable deprefix, so that the processed and logged output should be:

"output": "Assistant\nHomeopathy is a form of alternative medicine that has been practiced for centuries.