Issue #2972880 by andileco: Enable suffix and prefix on Highcharts Y-Axis labels without ruining formatting

Issue #2962561 by andileco: Work to eliminate the charts.pages.inc file by adopting a stronger OOP approach

Remove ModuleSelector, which was no longer being used in favor if plugins; make adjustments to the Chart Attachment display plugin to make it inherit more from Core; make some title and legend positioning changes for Highcharts

Issue #2953912 by andileco: Undefined index exporting in Drupal\charts_highcharts\Plugin\chart\Highcharts->buildVariables line 205

Issue #2953102: Highcharts pie/donut charts should be able to accommodate one node (or an aggregation) with multiple fields

Make Charts API Example module use plugins, since the render files were deleted. Now more closely resembles Charts Blocks.

Removed render classes in favor of plugins, which had taken over the functionality already. Also renamed the plugins to be more consistent.

c3 plugin name was not consistent with the renderer and was preventing c3 charts blocks from showing