Properly guard commands in the SyGuS API (#8390)

Properly guard commands in the SyGuS API (#8390)

This commit ensures we don't segfault in the SyGuS API (for non-text inputs) if the option sygus is not set to true. It also renames mkSygusVar to declareSygusVar for consistency with the sygus input format.

For SyGuS API inputs, we now use the option sygus to true instead of setting the language to sygus2.

This furthermore changes a few details in set-defaults regarding the relationship between the language, the sygus option, and when to apply default options for sygus.

It also adds a unit test for checkSynth.