Setting fillOpacity could change dot to default fill instead of stroke

Setting fillOpacity without also setting fill has no effect, so we could change the default for dot such that this

Plot.dot(cars, {x: "displacement (cc)", y: "economy (mpg)", fillOpacity: 0.2}).plot()

is equivalent to this

Plot.dot(cars, {x: "displacement (cc)", y: "economy (mpg)", fill: "currentColor", fillOpacity: 0.2}).plot()

For hexbin, that would mean you could say

Plot.plot({
  marks: [
    Plot.dot(
      cars,
      Plot.hexbin(
        { fillOpacity: "count" },
        { x: "displacement (cc)", y: "economy (mpg)" }
      )
    )
  ]
})

without having to set the fill option, too.

Start at the Plot.dot entry point and inspect how default fill and fillOpacity options are resolved; no file or test path is named in the issue. Use the two dot examples and the hexbin example as behavioral checks, with completion meaning fillOpacity alone produces the intended currentColor fill behavior.