Add Node.js esm entry point

[sebamarynissen]

This PR adds a Node.js esm entry point for Vuex. Currently it is already possible to do

import Vuex from 'vuex'

on Node 13.2 and up, but unfortunately it's not possible to do

import { mapGetters } from 'vuex';

because Node doesn't support named imports from CommonJS (note: it actually does, but only if the imported cjs file adheres to some rules, such as using exports.mapGetters). This PR adds support for named imports on Node as well.

While it is possible to directly use dist/vuex.esm.js, this PR does not use that file but chooses to use an esm wrapper file as to avoid the dual package hazard.

Why?

Node.js now fully supports esm so let's encourage the migration process by improving interoperability of the code we write. I've created this PR in the wake of writing vue-esm-loader which allows importing .vue single-file components on Node without a build step. I noticed that I was unable to test my Vue components because they were using

import { mapState } from 'vuex';

export default {
  computed: {
    ...mapState(['state']),
  },
};

and hence decided to add this functionality to Vuex.

Related: chaijs/chai#1340

[kiaking]

Thanks a lot on the PR! I've tested this at my environment too and it's working. Nice one 👍

I left a comment on not referencing src folder. Please have a look 🙏