Mark events internal to a machine

[tkvw]

Created this issue on request of David following this question on discord

Feature request/Typescript

I am using a callback actor which uses sendBack to the parent machine, but to handle the events in the machine I need to add the events in the setup events type. This is fine, but will also expose these events as being valid events to be send to the machine by an outside actor.
Maybe an additional type internalEvents on the setup type?
something like:

type TEvents = ..
type TInternalEvents = TEvents | ...
setup({
   types: {} as {
       events: TEvents,
       internalEvents: TInternalEvents
   }
})