className option for plot, facets, axes and marks

[Fil]

supersedes #126

very ugly test https://observablehq.com/@fil/plot-classname-253

[Fil]

https://observablehq.com/@mcmcclur/a-plot-selection-hack by @mcmcclur also wants class names, though it's not totally clear to me if this pull-request corresponds to Mark's use-case.

[Fil]

if there is interest, we should redo this as a common style (#490)

[RLesser]

Is there an issue for this beyond this pull request? Happy to make one if not or just start another pull request for className implemented as a common style. I'd find it fairly useful in the scenarios where I have multiple mark groups of the same type, and want to reference only one of them in post processing without relying on the order of the marks and filter statements.

[Fil]

@RLesser There wasn't, I've opened #1002