Consider migrating from vuex to pinia

https://github.com/vuejs/vuex:

> The official state management library for Vue has changed to [Pinia](https://pinia.vuejs.org/). Pinia has almost the exact same or enhanced API as Vuex 5, described in https://github.com/vuejs/rfcs/pull/271. You could simply consider Pinia as Vuex 5 with a different name. Pinia also works with Vue 2.x as well.

There is no active development anymore on vuex. As the usage of Pinia is very similar for Vue2 and Vue3, this would already help a bit towards migration to Vue3.

Also needs adjustments of hal-json-vuex (=potentially forking/renaming the package to hal-json-pinia). Would be a quick win compared to a complete [rewrite of hal-json-vuex](https://github.com/ecamp/hal-json-vuex/issues/277).