Correct usage of Iterator and Iterable interfaces

[ma2ciek]

As a follow-up to the #755 issue, we need to fix incorrect usages of these interfaces. If we go someday with the #1415 it will save us some time in fixing it anyway (TS is very accurate in the checking these interfaces). And it's easier to fix bugs in small PRs.

First of all, I'd redefine a little bit the rule that's in the #755 (comment).

1. Use the Iterable interface for classes that implement the [Symbol.iterator]() method.

2. Use the Iterable interface for function / method paremeter types. Unless you're sure that the function takes the Iterator (like the first( iterator ) function in ckeditor5-utils).

3. Use the Iterable interface for generators. They implement actually the IterableIterator interface, but we don't use the code in such a way AFAIK, so it should be safe.

4. Use the Iterator interface as a return type of the [Symbol.iterator]() method.

5. Use the Iterator interface for custom iterators.

[ma2ciek]

As an easy-to-remember way I'd go with TS interfaces:

interface Iterator<T> {
    next(value?: any): IteratorResult<T>;
    return?(value?: any): IteratorResult<T>;
    throw?(e?: any): IteratorResult<T>;
}

interface Iterable<T> {
    [Symbol.iterator](): Iterator<T>;
}

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