Make MarkupExtension an interface

I believe that making MarkupExtension an interface enables developers to work with (Static/Dynamic)Resources more flexibly. In my case it has hurt the creation of ResourceDictionaries with simple types like Double (e.g. font sizes or the like) or fonts in general.

The implementation of the class is already really close to that of an interface so it should not be too much work to make the change.

Example:
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```