Improve bandwidth calculation for single-value violins

[etpinard]

From @alexcjohnson in #2775

But if we think this is a real use case we should use other violins on the same subplot to pick a bandwidth that's reasonable for the whole collection, like we do with histograms

[cpsievert]

This seems to cause a reasonable amount of confusion for folks working with real data, see

plotly/plotly.R#1476
plotly/plotly.R#1462

A minimal example

https://codepen.io/cpsievert/pen/xBdxvo

A vertical line at the singular value seems like the right way to handle this, it's also consistent with ggplot2: