Refactor to use ColumnSpacing for cell borders

[symbiogenesis]

This would allow the BorderThickness to accurately set the real thickness.

When borders are around each cell, then the borders are effectively doubled. Setting a thickness of 1 really produces a border with a width of 2.

The performance after this change in unaffected.

[symbiogenesis]

@akgulebubekir What do you think about this? It is probably an improvement, at least in terms of correctness, as long as there aren't any regressions of any kind. Because setting odd-numbered thicknesses isn't possible, currently.

For performance, I haven't actually tried benchmarking the before/after with speedscope or anything.

For maintainability, this adds code and an event subscription, but the code seems slightly more readable. There might be a way to refactor it further for simplicity.

At some point if upstream MAUI performance improves, we could evaluate using a real Border control to keep things simple and maintainable. (I have been part of the effort to improve upstream MAUI performance, and progress is steady)

If you like this, let me know, and I will fix the merge conflict.