Align Slider API with SwiftUI

[stackotter]

The SwiftCrossUI Slider API differs from SwiftUI's Slider API in a few ways, and it's kinda annoying when transitioning code between the two frameworks.

SwiftUI has a value: label for its binding parameter, whereas SCUI's is unlabelled. SwiftUI also supports ranges with the in: parameter, whereas SCUI doesn't.

We should align the APIs so that transitioning code isn't needlessly annoying.

[stackotter]

#84 is a sub-issue of this. It covers the in: parameter part.