Implemented Extends and Implements extension methods for the Type class

[abjerner]

The syntax or wording around the IsAssignableFrom method feels a bit weird, so this PR introduces Extends and Implements extension methods to be used as alternatives.

While the two methods do the same, the idea is that the Extends method can be used to check whether a System.Type extends a given class, and that the Implements method can be used to check whether a System.Type implements a given interface.