[Views] - shadow modifier method

[suragch]

In SwiftUI you add shadow to a View with the shadow modifier method:

Rectangle()
    .fill(Color.purple)
    .shadow(color: .gray, radius: 10, x: 0, y: 10)
    .frame(width: 200, height: 100)

Note that this is a method on View and not on Shape. See #27. For that reason, shadows can be applied to any view. So in Flutter, we can make this a composable widget:

Shadow(
  color: Colors.grey,
  radius: 10.0,
  x: 0.0,
  y: 10.0,
  child: AnyWidget(),
)

Tasks:

• Create SwiftUI examples
• Implement Flutter widget with doc comments
• Create Flutter examples
• Add golden tests