Provide a way for implementations to announce supported features

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.

まず、issue で指定されている $psEditor API と、powerShell/getEditorContext および powerShell/showChoicePrompt の処理を読みます。エディターがサポートしている requests を公開でき、拡張モジュールが requests を送信する前にその情報を照会できれば完了です。