Support SVG patterns

[Fil]

fill: "url(#hatched)" should be detected by the color matcher, and treated as a constant rather than a field

reference: https://www.w3.org/TR/SVG11/painting.html#SpecifyingPaint

[mbostock]

I think this should work as a regex

/^\s*url\(.*\)\s*$/i

[Fil]

Example with textures.js https://observablehq.com/@fil/qatar-population-pyramid

[Fil]

https://texturemap.org/ seems interesting also

[Fil]

Cheysson example https://observablehq.com/@fil/cheysson-plot

[mbostock]

And https://riccardoscalco.it/textures/