[TS Followup] Change mixins style

[arkflpc]

There are two styles of mixins suggested by TypeScript Handbook: suggested and alternative one.

Currently, the usage of mixins in ckeditor5 looks like:

export default class MyClass { ... }
mix(MyClass, EmitterMixin);

After migration to TypeScript, we could use the alternative pattern and translate the above into:

class MyClass { ... }
mix(MyClass, EmitterMixin);
interface MyClass extends Emitter {}
export default MyClass;

However, when we want to mix both DomEmitterMixin and ObservableMixin:

class MyClass { ... }
mix(MyClass, DomEmitterMixin);
mix(MyClass, ObservableMixin);

// won't work, because listenTo must be identical in all super-interfaces:
// interface MyClass extends DomEmitter, Observable {}

// But type intersection works differently, so let's use that:
type ObservableDomEmitter = DomEmitter & Observable;
interface MyClass extends ObservableDomEmitter {}
export default MyClass;

This is doable, but seems like a hack. I propose to use the pattern from handbook:

export default class MyClass extends ObservableMixin( DomEmitterMixin( Object ) ) { ... }

This could be made backward-compatible (XyzMixin functions would have all the methods also exposed on the function object itself, at least for the existing mixins).

[arkflpc]

Check if tests are not noticeably slower.

[arkflpc]

Add more info in ChangeLog (i.e. It's now recommended to use ... ).