fix work item indices. Add sycl indexing config.

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/emission.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/literals.py

src/pystencils/backend/platforms/__init__.py

src/pystencils/backend/platforms/sycl.py

src/pystencils/config.py