Introduce tile-grid?

[frewsxcv]

https://github.com/t-rex-tileserver/t-rex/tree/master/tile-grid

[frewsxcv]

and/or https://github.com/georust/tilejson

[pka]

https://github.com/t-rex-tileserver/t-rex/tree/master/tile-grid

This code has moved to https://github.com/pka/tile-grid/tree/main/src/wmts