feature: add default interactive tips

[jamesscottbrown]

"Default interactive tips"/tooltips are a convenient feature in Observable Plot: https://observablehq.com/plot/features/interactions

(this is one of the issues surfaced by #72)

[gka]

duplicate of #89

[jamesscottbrown]

It's not quite a duplicate. There are 2 different ways to add tips to an ObservablePlot plot: explicitly, by adding a Tip mark, or implicitly by setting tip: true on the mark that tooltips should be added to (and optionally also setting channels to specify which fields should be listed in the tooltip). Issue #89 is about adding the Tip mark (which is probably a prerequisite for adding default tooltips)

Default interactive tips created by tip: true:

Plot.plot({
  marks: [
    Plot.dot(olympians, {
      x: "weight",
      y: "height",
      stroke: "sex",
          
      channels: {name: "name", sport: "sport"},
      tip: true
    })
  ]
})

Explicitly created tip mark:

Plot.plot({
  marks: [
    Plot.dot(olympians, {
    x: "weight",
    y: "height",
    stroke: "sex",
  }),
  Plot.tip(olympians, Plot.pointerX({
    x: "weight",
    y: "height",
    channels: {name: "name", sport: "sport", sex: "sex"}
  }))
  ]
})