Hook up PSES events to PSEventManager

Right now the only way to register event/request handlers is to override them via IMessageHandlers from the component registry. This has a few issues

1. As far as I can tell this only allows you to override handlers, not add to them. Doing this as part of an extension module would break existing functionality.

2. These delegates are invoked from a thread that doesn't have a default runspace. This means if the delegate is a converted script block it will fail (and crash PSES)

I propose we add the following:

1. Function or cmdlet Register-EditorEvent. This should work like Register-EngineEvent. It would take a SourceIdentifier as the event name and script block to register as a PSEventSubscriber. Ideally this would also include argument completion for event names.

2. A class that holds constants with event names (see PSEngineEvent)

3. Additional logic to existing handlers and other events to check for relevant event subscribers and if found generate the event.

4. If the event/handler typically returns a value, a property would need to be added to the EventArgs to handle output (PowerShell eventing doesn't handle delegate output)

This would solve a few issues

1. Extension modules could register additional handlers the same way in PowerShell and compiled languages

2. Eventing should take care of all the runspace management and queuing of commands that would normally make this a huge pain. This would also allow the events to be ran in between sequence points of an in progress command.

3. Allow multiple handlers for a single request/event

首先跟踪组件注册表中的现有 IMessageHandlers，并将提议的 Register-EditorEvent 行为与 Register-EngineEvent、PSEventSubscriber 和 PSEngineEvent 进行比较。定义事件名称常量、订阅者注册、处理程序分派、runspace 排队以及 EventArgs 输出行为；完成的标准是多个 PowerShell 和编译型语言处理程序可以共存，而不会替换现有处理程序。