Allow light-dark() function in color values

[tmcw]

I would like to create a mark like this:

Plot.areaY(stats, {
  y: "y",
  x: "x",
  fill: "light-dark(blue, lightblue)",
})

However, this does not work because isColor checks for a bunch of color formats as well as the color-mix function, but not for light-dark(), which makes it possible to automatically support color schemes.

I think that adding light-dark to that method would be all that's necessary.

[mbostock]

Done in #2412, but as a workaround you can use a custom property such as var(--color):

Plot.plot({
  style: "--color: light-dark(steelblue, orange); color-scheme: light dark;",
  marks: [Plot.barX(alphabet, {x: "frequency", y: "letter", fill: "var(--color)", sort: {y: "-x"}})]
})