ProxyWatchers small declaration issue

[zeroinformatique]

Hello, thanks for this library, it's very useful !

I use the ProxyWatchers in my code, but since I create the proxies only once and use an interface to access them, I had to fetch the type manually with:

import { extractVuexModule, createProxy } from "vuex-class-component";
import { ProxyWatchers } from "vuex-class-component/dist/interfaces";

Hitting "/dist/interfaces" doesn't really pleases me. Could you move it to the root declaration?

Also I noticed that in SubScribeActionCallback, "before" and "after" functions are both mandatory, they should be optional (either before, after, or before + after)