Feature request - make LineChartRenderer inheritable

I needed to subclass `LineChartRenderer` and override its method `drawCircles(context:)`. It is private but ok, I can override `drawExtras(context:)`.
The problem is these methods involve a lot of private/internal methods which are not accessible. So instead of overriding a single method I need to subclass `LineRadarRenderer` (superclass of `LineChartRenderer`), copy a variable, 2 internal methods, 3 extensions (which are not accessible outside this module) and a global function.

Linked problems:
https://github.com/danielgindi/Charts/issues/3258
https://github.com/danielgindi/Charts/issues/5081

Start with LineChartRenderer.drawCircles(context:) and drawExtras(context:), then trace the private and internal methods, extensions, variable, and global function involved in their implementation. Compare the linked issues for context. Done means LineChartRenderer can be subclassed to customize circle drawing without copying inaccessible LineRadarRenderer internals.

Written by the indexing model from the issue text.