Add shadow classes as a way to extend sealed classes.

We have a way to re-implement non-overridable (sealed) methods, fields and properties using the Shadows keyword.
But on class-level we do not have a way to override non-inheritable (sealed) classes.

I'd like to see the Shadow keyword for inheritance too, making the following syntax valid:
`Public Class MyClass`
` Inherits Shadows System.String`
`End Class`

The inherited class should 100% act like system.string, but with the possibility to reimplement (shadow) all public members.

An example of the internal representation of the shadowed class can be seen with the extension "ShadowClasses" (link: [https://marketplace.visualstudio.com/items?itemName=theoverweij.ShadowClasses-Extension-For-VB](url) )