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.

首先阅读 issue 中提到的 $psEditor API，以及对 powerShell/getEditorContext 和 powerShell/showChoicePrompt 的处理。完成的标准是：编辑器能够公开其支持的 requests，并且扩展模块能够在发送 requests 之前查询这些信息。