Add themed initalizers

_Available for Apple platforms only_
# Example
```swift
UIColor(light: UIColor(...), dark: UIColor(...))
SwiftUI.Color(light: SwiftUI.Color(...), dark: SwiftUI.Color(...))
UIColor(light: Color(...), dark: Color(...))
SwiftUI.Color(light: UIColor(...), dark: UIColor(...))
SwiftUI.Color(light: Color(...), dark: Color(...))
```