Provide a way for implementations to announce supported features

[SeeminglyScience]

Right now as far as I can tell there isn't a great way to tell if the editor supports a specific request. For example, if a consumer of the $psEditor API wants to request editor context with $psEditor.GetEditorContext() the only way to know if the editor supports it is to try and possibly fail.

Making things a little more complicated is every request can act a little differently. For instance, powerShell/showChoicePrompt just returns null, where as powerShell/getEditorContext will throw a NullReferenceException.

We should have an API where the editor explicitly states which requests they support. That way an extension module can send only the requests they know will work.