Is your feature request related to a problem? Please describe.
When pie chart sections are adjacent, borders appear doubled/thicker because the current borderSide property applies the same border to all sides of a section. There's no way to control which sides have borders.

Describe the solution you'd like
Add a new PieChartSectionBorder class that allows controlling each border side independently:

• outer: the outer arc (furthest from center)
• inner: the inner arc (closest to center)
• left: the left radial line (start of the section)
• right: the right radial line (end of the section)

This would allow users to use .arcsOnly() to only draw borders on the arcs, avoiding the doubled borders issue when sections are adjacent.

Describe alternatives you've considered

• Using sectionsSpace to separate sections, but this changes the visual appearance
• Manually adjusting border widths, but this doesn't solve the fundamental issue

Additional context
This is useful for creating clean pie charts with borders where sections should appear seamlessly connected without visible doubled borders between them.