allow for Stan engine from rstan package

[kevinushey]

This PR adds a provision for calling rstan::eng_stan() when running a Stan chunk, when such an engine exists.

(The intention is to move the Stan engine implementation to the rstan package, so it can evolve as required when the package evolves)

[yihui]

I'll hold on merging this PR till eng_stan actually exists in rstan.

[CLAassistant]

All committers have signed the CLA.