Add support for UICornerConfiguration in iOS 26

[WillBishop]

This is a tiny PR that adds supports for UICornerConfiguration. If it is set on SurfaceAppearance, any other property like cornerCurve or cornerRadius is ignored.

[WillBishop]

(the checks fail because it's not running against the Xcode 26 beta)

[scenee]

Thanks for this PR! Using the compiler(>=6.2) compilation condition like this should allow the checks to pass.

    #if compiler(>=6.2)
    @available(iOS 26.0, *)
    public var cornerConfiguration: UICornerConfiguration? {
        get { _cornerConfiguration as? UICornerConfiguration }
        set { _cornerConfiguration = newValue }
    }

    private var _cornerConfiguration: Any?
    #endif