It shouldn't be neccessary as instance.
It should be available from outside

- vuex and vue/compat are added to peerDependencies,
they are not needed as regular dependencies
- vue and vue-template-compiler (the latter needed for
vue 2, but not vue 3) could be removed as dependencies
- to be able to run the library in a vue 3 project,
vuex is required in ^v4

to be able to remove vuex as a direct dependency,
we need to remove its usages in the library

to be able to remove vue as a dependency,
we need to remove all its usages

these are needed for running the tests

- since state.items is an object, using `findIndex` on
it throws an error
- instead, we can just use `delete` to delete the item