fix(styles): isolated point style overrides

[nickofthyme]

Summary

This change applies a hierarchy to the (AreaSeriesStyles|LineSeriesStyles).isolatedPoint styles. This is to prevent changing values in one to override the other. If the consumer applies a style to the points they would expect the style to transfer to all points.

The hierarchy would be (in order of highest priority):

• (AreaSeriesStyles|LineSeriesStyles).isolatedPoint
• (AreaSeriesStyles|LineSeriesStyles).point
  • This was the missing piece in the current logic
  • Skips visible override
• Theme.(areaSeriesStyle|lineSeriesStyle).isolatedPoint

Details

I don't think the onus of this styling should be on the chart consumer to always supply a backup style to isolatedPoint whenever they are setting the point style.

Issues

fix #2277

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios
• The proper documentation and/or storybook story has been added or updated
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)
• Visual changes have been tested with light and dark themes

[nickofthyme]

@markov00 could you take another look at this?

[nickofthyme]

Comment from #2277 (comment)

There is one more issue related to that:
isolated points in Stacked areas are not shown if the consumer legitimately sets the areaPointStyle visibility to false. This with our default theme (EUI) isolated point on these time of charts are interactive but not visible:

At this point, an isolated point should inherit the style of a point except for the visibility that should always be true, except if explicitly disabled in the isolated point style.

[markov00]

Seems to work fine for me, thanks!