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

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

Issue #2926078 by lbaran: Charts API Example has issue with ModuleHandler.php and issue #2926077 by lbaran: In Default chart configuration, get AttachedAssets.php notice. Also includes misc. changes to HighchartsChartsRender by andieco

Enable donut option for Highcharts and Google Charts (there's still work that needs to be done to these for multiple Chart attachments). Also adds a message to the Charts API Example module if the Charts default settings have not been configured.

Created a ChartsInterface as a place to define constants. Also moved the API example module into the modules folder. And fixed code formatting in some places.