-t custom toplevel option

-t custom toplevel option

- Add -t FLAG to specify custom toplevel (arity 0 predicate)
- Default toplevel is 'repl' if -t is not specified
- Using `-t halt` achieves original goal of guaranteed termination
- Custom toplevels enable flexible exit strategies (e.g., server mode)
- Update help text to document -t flag

Examples:
  scryer-prolog -t halt program.pl    # Exits after execution
  scryer-prolog -t my_repl program.pl # Custom REPL
  scryer-prolog program.pl            # Default REPL

Co-Authored-By: J.J.'s Robot <[email protected]>