Use LinkedHashSet for ObservableSet default implementation.

[pr-1]

Flutter sky_engine by default uses LinkedHashSet for Set implementation.
https://api.flutter.dev/flutter/dart-collection/LinkedHashSet-class.html

Why don't we use LinkedHashSet as default for ObservableSet constructors ?
https://github.com/mobxjs/mobx.dart/blob/master/mobx/lib/src/api/observable_collections/observable_set.dart

This causes confusion in analogy of Set and ObservableSet. Ideally they should behave same with just an observable wrapper.
But if you use the default ObservableSet it will not retain the order of iteration. on addAll() or any other method.
Also I would add these things to the docs if we don't change this behaviour.

I am aware of the alternate ObservableSet.linkedHashSetFrom provided separately in the api but ideally this should be the default one in my opinion.

[fzyzcjy]

LGTM, what about raising an issue or PR :)

[pr-1]

Happy to do that :)

[amondnet]

#869