GraphState and Stim simulator backend prototyping

[shinich1]

Describe the feature you'd like
Explore whether a fast Clifford (Pauli measurement only) pattern simulation backend can be suitable.
It would require an interface to configure the noise model in the simulation, similarly to the NoiseModel interface for DensityMatrix backend, but limited to random Pauli errors.

1. add example implementing pattern simulation with graphix.GraphState
   Implement a new pattern simulation backend on a example file, based on graph-state simulator.
   During the simulation, throw an error if non-Pauli measurement is detected. Output of the simulation can simply be graphix.GraphState object.

2. pattern simulation with stim for very fast sampling of results
   Add an example file, and in the example file, write up a transpiler of Clifford pattern into a stim circuit. Demonstrate small-scale error detection or correction.