Allow custom 'vuex bindings' to be displayed in ComponentInspector

[jdoubleu]

What problem does this feature solve?

So far (computed) properties bound with mapState Vuex Component Binding Helper will be displayed as "vuex bindings" in the component inspector.

The helper exports a function with a property named vuex. In the devtools this property is checked for all computed properties (see here).

However, it is not possible to mark a (computed) property as "vuex bindings", if a custom userland wrapper (as supposed here) does not return a function.
This especially conflicts if the property has a computed setter and thereby is an object (see also).

This would make it possible to show another developer that a property is actually bound to the vuex state in one's app.

What does the proposed API look like?

The internal API only needs to check if the property is a "function" or an "object" in https://github.com/vuejs/vue-devtools/blob/dev/src/backend/index.js#L714.

One side-effect is that even in the component itself a property could be marked as "vuex binding". One could also want this, following the Form Two Way Binding example.