llama: non-bool `IncrementalStringFinder` and `AntipromptManager`

[iboB]

To properly manage antiprompts users must have two options available:

• break after the antiprompt was generated
• backtrack to when before the antiprompt was generated and continue from there

Currently only the first option is supported.

To properly support the second one, the information where to backtrack to should be available

[iboB]

Related to #132 but not blocked by it. The information should be available, the backtracking itself is out of scope for this issue.