Add small caret/arrow to indicate which mark a tooltip belongs to

[joelostblom]

Now that #959 was merge (thanks @marshallpete !), I believe an interesting follow up would be to more clearly indicate which mark the tooltip belongs to. For example, sometime the tooltip is a bit away from the mark which makes it hard to see which mark it belongs to (as seen in https://vega.github.io/vega-tooltip/vega-lite-examples.html). I believe a small caret/arrow that points on the mark could be helpful, similar to what observable are doing here https://observablehq.com/plot/marks/tip. Thoughts?

I also wonder if this new mark anchoring could help advance vega/vega-lite#6851 or if that is still quite different since it is about how tooltips are triggered rather than where they show up?