User configurable tile styles / url templates

[vorbei]

in https://github.com/neocarto/bertin/blob/main/src/layers/tiles.js the 'style' parameter only supports string type, and not user extendable.

for a custom style/url, please consider support Object parameter like

{
     name: "openstreetmap",
     provider: "OpenStreetMap contributors",
     url: (x, y, z) => `https://tile.openstreetmap.org/${z}/${x}/${y}.png`,
   },