Isolated point styles override user defined point styles

[nickofthyme]

Describe the issue
With the changes from #2004 to expose isolated point styles, this inadvertently overrides provided styles such as point.radius only for the isolated points

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/romantic-tess-gwvxw4?file=/src/App.tsx
2. Notice the style difference between the isolated points (i.e. null, value, null).

Expected behavior
We should use merge the styles from LineSeries.lineSeriesStyle.point with LineSeries.lineSeriesStyle.isolatedPoint. Same for
AreaSeries we should merge AreaSeries.areaSeriesStyle.point with AreaSeries.areaSeriesStyle.isolatedPoint.

Screenshots

Version (please complete the following information):

• Elastic Charts: v58.0.0

Kibana Cross Issues
elastic/kibana#172995

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• Every related Kibana issue is listed under Kibana Cross Issues list and the kibana cross issue tag is applied

[markov00]

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.