z-aware pointer

[Fil]

Demo: https://observablehq.com/@observablehq/pointer-z-1671

Questions:

• Do we always follow an explicit z? ✓
• Do line marks (and area, etc.) set z when the stroke or fill option is specified? ✓
• Is this something we want to opt-in more explicitely (e.g. zPointer: true)? NO

Done:

• [x] expose the series for viewof, not just one data point.
• [x] create the groups in an initializer

https://github.com/observablehq/plot/assets/7001/6dafd782-6c06-4304-9eb2-2178af847531

closes #1574

maybe also part of a solution for #1621?

[Fil]

Note: in order to minimize the diff, I did not wrap the call to initializer(options, ƒ). To do before merging.

[Fil]

rebased

[mootari]

It would be great to have this in Plot! 🙏🏻 What remaining issues need to be resolved?