[Dexter] Add condition check to state nodes (#203847)
This patch enables the ability for state nodes to check conditions,
meaning they will be active only if the condition is met.
Condition evaluation is somewhat language specific; we directly check
whether the value of the evaluated expression is "true"
(case-insensitive), which works for the languages we actually use Dexter
with, but may require generalizing in future.
We also cache conditions as they are evaluated; each time we step, we
clear all cached conditions for the current frame and any expired
frames, but we keep the cached conditions for any frames rootwards from
the current frame; this prevents us from unexpectedly exiting out of a
callee frame because of debug info not surviving a stack unwind; if the
early exit is desired, an !and{at_frame_idx, condition} under the lower
frame may suffice.
