**Why?**
The [Web IDE file mirroring][1] will need to register a plugin,
similar to the terminal plugin.

Since plugins are a [native Vuex thing][2], it is helpful keeping
the consistent terminology and interface (even though we
aren't using the `plugins` property yet).

**References:**
[1]: https://gitlab.com/gitlab-org/gitlab-ee/issues/5276
[2]: https://vuex.vuejs.org/guide/plugins.html