feat: refactor ProcessPromise internal state markers

[antongolub]

We use several private fields to describe the process stage, but we'd like introduce explicit state machine contract

Expose this

_halted
_piped
_output // which means that is fulfilled

via smth like

type ProcessStage = 'halted' | 'pending' | 'fulfilled' | 'rejected'

stage(): ProcessStage {
  if (this._halted) { return ... }
  // ...
}

[easymikey]

I'm ready to implement

[easymikey]

@antongolub, is it supposed to get state from the outside?
And a few more question:

1. Is _run state equal to fullfilled?
2. Are functions like isHulted still needed?
3. Are field like '_piped`, '_output' and other still needed or cat _state field replace it?
4. Why don't use #run instead of private _output ?