Add `decompose_errors` flag to `dem_from_kernel`

[eliotheinrich]

Required prerequisites

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Describe the feature

For integrating detectors/logical observables into CUDA-Q Libraries, we need access to error-decomposed (i.e. graphlike) DEMs. The current path called by dem_from_kernel has hardcoded decompose_errors=false in StimCircuitSimulator.cpp:generateDem; since the actual decomposition is ultimately relegated to stim, we can get this functionality by just wiring up a flag.