feat: enable default tooltip for boxplot outliers

[joelostblom]

This is a bit of a issue/discussion, but I wanted to show which change I think will work so I made a PR right away. I think it is quite useful to see hover info on boxplot outliers to get more into about these points. Having this enable by default facilitates getting this info right away without adding a tooltip encoding and is in line with the default tooltip added for the box and whiskers. One drawback could be that the tooltip gets cluttered if there are a lot of columns in the dataframe, and that it might not be possible to turn it off completely (but this is also the case for the box and it is easy to set it to a single field).

[domoritz]

Please run the formatter. Otherwise looks good to me.