Add support for OS Open Zoomstack

Nice new OS vector tileset:

https://www.ordnancesurvey.co.uk/business-and-government/products/os-open-zoomstack.html

https://www.ordnancesurvey.co.uk/demos/open-zoomstack/index.html

Main code is at:
https://www.ordnancesurvey.co.uk/demos/open-zoomstack/js/map.js
and shows this is just a standard MVT definition.