Refactor DeclareSygusVarCommand and SynthFunCommand to use the API. (#5334)

Refactor DeclareSygusVarCommand and SynthFunCommand to use the API. (#5334)

This PR is part of migrating commands. DeclareSygusVarCommand and SynthFunCommand now call public API function instead of their corresponding SmtEngine functions. Those two commands don't fully initialize the solver anymore. Some operations in SygusInterpol::solveInterpolation, which creates an interpolation sub-solver, depend on the solver being fully initialized and were affected by this change. Those operations are now executed by the main solver instead of the sub-solver, which is not fully initialized by the time they are needed.