Fix AsyncComputed type.

[rudolfbyker]

PluginFunction<never> should be PluginFunction<void>, otherwise the following code will not satisfy TS checks:

import Vue from "vue";
import AsyncComputed from "vue-async-computed";
Vue.use(AsyncComputed, {})

never means "this function never returns", while void means "this function always returns undefined"

[boukeversteegh]

Great! Seems like a simple fix to a problem I've also encountered. Can we merge this? @foxbenjaminfox

[rudolfbyker]

More than a year later. Very simple fix. Please merge this.

[cdrini]

Hello @rudolfbyker , I'm helping maintain this repo now. Any chance you'd be able to revisit this PR now that the project's migrated to Vue 3? I reckon all the types could use an overhaul possibly.

[rudolfbyker]

Sorry, I no longer use this project. But feel free to still merge or close my PR as you see fit.

[cdrini]

No worries, thanks @rudolfbyker ! I'll see if the types are still valid in Vue 3 when I get the chance.