[Feature Request] Allow controlling borders on each side of pie chart sections independently

**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.

No file or test path is named. Start by locating the pie-chart implementation and its existing `borderSide` handling, then inspect related tests or examples. Done means a `PieChartSectionBorder` API independently controls outer, inner, left, and right borders, including an arcs-only configuration that avoids doubled borders between adjacent sections.

Written by the indexing model from the issue text.