feat: refactor `ProcessPromise` internal state markers

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

Expose this
```ts
_halted
_piped
_output // which means that is fulfilled
```

via smth like
```ts
type ProcessStage = 'halted' | 'pending' | 'fulfilled' | 'rejected'

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