[TS MVP] Rewrite ckeditor5-utils package to TypeScript

[arkflpc]

We need to start moving CKEditor5 to TypeScript somewhere, and ckeditor5-utils is the perfect package to start with. That's because:

1. It doesn't depend on anything else, so we won't introduce a TS package that depends on JS one.
2. Most classes and functions are independent.
3. It's quite stable, so the code won't diverge much from the master branch (it takes several tasks to merge it there).

Because we don't have any tooling supporting TS right now, let's keep the code on feature branch.

Exploiting the full potential of statically typed classes like Emitter or Observable can be tricky to do at the first attempt, so I wouldn't overcomplicate that. We can get back later. For instance, when rewriting ckeditor5-engine.

Part of #11708.