Plot.text halo option(s)

[Fil]

It's a CSS design question + an API design question.

stroke-width: 4;
fill: none;
stroke: white; 

The stroke: white is controversial, since we don't know that currentColor is black (or any dark color).

API design question: do we need three options (halo: true, haloStroke = "white", haloWidth = 4)?

(This can't be done by applying two text marks, because the halos are also there to fight overlapping texts.)

Ref. #27 and https://observablehq.com/@mbostock/inequality-in-american-cities