This introduces a new mechanism for evaluating lookbehind assertions by
adding four new bytecode opcodes: SetStepBack, IncStepBack,
CheckStepBack, and CheckSavedPosition.
These opcodes replace the previous GoBack-based approach and enables
correct handling of variable-length lookbehind patterns,
where the match length cannot be known statically.
Track lookbehind greediness in the parser and propagate it to bytecode
generation. Allow controlled backtracking in lookbehind bodies while
avoiding incorrect captures during step-back execution.
Partially fix issue: #3459
