Fix sexpr bug with AST output language. (#6329)

Fix sexpr bug with AST output language. (#6329)

When a command is invoked, its result/status may be printed depending on its current verbosity, which (for probably an outdated reason) is stored in SMTEngine. One of my previous PRs modified the SMTEngine to return the verbosity as an sexpr. The modification works correctly when the output language is SMT2, but it breaks down with the AST output language, which prints sexprs in a different way. This PR fixes this bug by making SMTEngine return options as strings instead of sexpr.