TypeScript: Improve type definition to use optional chaining

[Kiikurage]

Optional Chaining is the syntax in upcoming ECMAScript and introduced in TypeScript v3.7.

In #240, I(Sorry, account is different but it's me) changed react-refetch's type definition so that unsafe access can be detected by TS compiler.

render() {
  if (!this.props.userFetch.fulfilled) {
    return <span>Loading...</span>
  }

  // Here, TS compiler knows "userFetch" is "FulfilledPromiseState" instance, because "uerFetch.fulfilled" is "true"
  return <span>{ this.props.userFetch.value.name }</span>
}

However, sometimes people want to simplify this by using optional chaining

render() {
  return <span>{ this.props.userFetch.value?.name || 'Loading...' }</span>
}

This PR improve type definition so that all PromiseState type have value field to use optional chaining for generic PromiseState type.

[ryanbrainard]

Thanks, this makes sense.

[ryanbrainard]

Released in https://github.com/heroku/react-refetch/releases/tag/v4.0.1-0

Going to let that bake a little bit and then we can push out v4 as GA