Fix cursor issue when hovering splitter

[BenjaminBini]

The custom cursor was not displayed correctly when hovering the splitter.
I'm not a Swift expert at all but I managed to make it work with this modifier.

[SteveBarnegren]

I've just tested this fix and it looks to be working well for me.

One small issue is that NSCursor.columnResize and NSCursor.rowResize are only available from macOS 15, but SplitView supports macOS 12, which is causing an error.

My app only supports macOS 15, so I've solved that by changing the platform to be .macOS(v15). To merge this in it would probably need to be called conditionally though, with some other option for older macOS versions.

[stevengharris]

Thanks for your comments and suggestion!