[WIP] chore(hierarchical-dependency-injection): Subscriber -> Observer

[wardbell]

WIP - Do not merge until the question below has been answered

Current heroes.service.ts code in the form of

new Observable<Hero[]>((subscriber: Subscriber<Hero[]>) => {...}); // (1)

That conforms to the type of the parameter for Observable constructor.

But it appears that Subscriber isn't supposed to be used in "public API"
and that the conventional thing to do (which works also) is to write.

new Observable<Hero[]>((observer: Observer<Hero[]>) => {...}); // (2)

I have asked those who know which is the preferred style ... (1) or (2) ... and if (2 ... as this PR would do), why is the arg to the ctor (and to the create method) typed as Subscriber?

[wardbell]

The question was answered (Paul Taylor) and the observer is preferred.