refactor: RxJS chains

[martinsik]

I went through the code base and refactored some RxJS usage where I thought it could be improved or where it was using old and deprecated notations. I'm adding comments where it's not obvious what I did there.

In general there are two larger changes:

• Replacing deprecated subscribe(v => void, func) calls with subscribe({ next: v => void, error: e => void}) Refine subscribe and tap signatures ReactiveX/rxjs#4159.
• Replacing Observable checks like typeof res.subscribe === 'function' with isObservable(res.subscribe).

All existing tests were passing I just added one more additional test for some sync behavior.

[ocombe]

@martinsik it looks great! You're really better at rxjs than I am 😁
could you rebase the branch on master so that the tests can run please? if it's all green then I'll merge it!

[iamnaumovnikita]

Guys, as I know the function isObservable should consume obseravble, but in your code it consumes subscribe function.
So, I think this code "isObservable(res.subscribe)" should be chnaged to "isObservable(res)" @ocombe and @martinsik please take a look

[martinsik]

@iamnaumovnikita You mean this line? https://github.com/ngx-translate/core/pull/1137/files#diff-5a524f36ea5ac706856e90f772633f7f76a3a11e3a55e39e576045fc7c20f967R30

I think you're right, this is incorrect. I'll check it.

[iamnaumovnikita]

I hve created PR #1411 can you review it, please? @martinsik @ocombe
Thank you very much!