Replace EE store extend with Vuex plugins property

The following discussion from !11481 (merged) should be addressed:

• @pslaughter started a discussion:

  Why do we apply the plugins here instead of using Vuex's plugins property?

  Using the plugins property would be ideal, but unfortunately it would take a bit of refactoring in CE to support injecting that. IMO, that's more appropriate for a follow up MR.