Add support for observablehq/plot

[DonJayamanne]

https://github.com/observablehq/plot

[DonJayamanne]

@randomfractals is this the package you were suggesting?

[RandomFractals]

yeah, @mbostock's new Plot lib: https://observablehq.com/@observablehq/plot

I have a set of demos you can check out on ObservableHQ: https://github.com/RandomFractals/Chicago-transportation-notebooks

Data summary notebook renderer I ported last month uses it: https://github.com/RandomFractals/vscode-data-table#data-summary--renderer-view