Provide initialiser for custom TableBorderSelector

[adJGraeser]

Currently there is no way to create a custom TableBorderSelector, since the initialiser of TableBorderSelector and TableBounds aren't exposed. This PR changes those to public. With that we can now create custom TableBorderSelector like this one:

extension TableBorderSelector {
    public static var firstHorizontalBorder: TableBorderSelector {
        TableBorderSelector { tableBounds, borderWidth in
            guard tableBounds.rowCount >= 1 else { return [] }

            return [
                tableBounds.bounds(forRow: 0)
                    .insetBy(dx: -borderWidth, dy: -borderWidth)
            ]
                .map {
                    CGRect(
                        origin: .init(x: $0.minX, y: $0.maxY - borderWidth),
                        size: .init(width: $0.width, height: borderWidth)
                    )
                }
        }
    }
}