`useDispatch`: deprecate mixin

Since Stimulus now got a dispatch() function built-in (see: https://github.com/hotwired/stimulus/pull/302) we should think about deprecating (and later removing) the useDispatch() mixin from Stimulus Use.

While it's still handy to have it included, so older apps can also use it, we should go through a deprecation cycle and start printing deprecation warnings to people still using it and let them know how they can migrate it.

Follow-up of #174

I agree that this should be part of 1.00.