className mark option should allow multiple classes (separated by spaces)

[awconway]

It seems that the className option for marks will only accept one class but should probably accept more as it is quite common for approaches like tailwind to use many classes. Example here:

https://observablehq.com/d/25a81df102bdaa88

[mbostock]

Yes, the validation of this option is currently too strict. We should relax it to allow multiple classes.