Allow install as mixin

[shrpne]

Fixes #105

Now it also can be used like this:

import {AsyncComputedMixin} from 'vue-async-computed'; // my component export default { mixins: [AsyncComputedMixin], data() { return {someValue: 123} }, asyncComputed: { myAsyncValue() {/* ... */} } }

Diff is pretty big, but it is because of whitespaces. In fact, there is only mixin moved to getAsyncComputedMixin and Vue.set replaced with vm.$set. So better to view with ignored whitespaces

Note: it may be breaking for users, who used it as const AsyncComputed = require('vue-async-computed') without interopRequire, because previously only default was exported and now both default and named are exported.

[foxbenjaminfox]

OK, thanks—this looks reasonable, I've merged it.

I'm in the middle of trying to adapt this plugin for Vue 3 compatibility, and so I'll probably be releasing a new major version (with Vue 3 support) sometime soon. I think I'll probably include this change as part of that release. (Seeing as that will be a major version in any case, that will take care of the concerns about changing the export style.)