feat(dfir_lang): add `last_iteration()` operator

[MingweiSamuel]

Un-windowing operator which takes only items produced on the final iteration of the loop block

[shadaj]

Of all the operators, this can probably come last. It is not in the Flo paper, since it would require either a) all input streams-of-streams to be bounded which is fairly atypical b) there to be a guarantee of fixpoint termination, which is difficult to prove automatically

[shadaj]

Oops, it is actually in the Flo paper, but only for (a).