Enable generator syntax for sub-queries

It would be nice if the following syntax would work:

rts = @from row in results begin
    @where row.trial >= 1
    @group row by (row.sid, row.trial) into trial
    @let filtered_trial = (i for i in trial if i.code in ["stream_1", "stream_2"])
    @where length(filtered_trial)>0
    @select {
        rt = last(filtered_trial).time - first(filtered_trial).time,
        response = last(filtered_trial).code,
        trial_length = last(trial).time - first(trial).time
    }
    @collect DataFrame
end

Start by reproducing the Julia query shown in the issue and inspect how Query.jl currently handles generator expressions inside grouped queries. Define completion as making that syntax execute successfully with the demonstrated filtering and selected fields, while preserving existing query behavior.