Vue 3 upgrade

[wolfjagger]

Related issues & PRs I can find: #1251 #1286 #1289 #1307.

Upgrad vue to 3.
Upgrade vue-related packages. This uses several beta packages, so I don't know how we feel about that.
Event API breaking changes (emits, modelValue, ...).
Change 'propsData' to 'props'.
Change 'scopedSlots' to 'slots'.
Update dev.js with createApp breaking change.
Adjust webpack build for vue 3 (change vue alias to esm-bundler, add environment define variables).
Set vue as external library so it's not bundled in.
Add prepare script (can remove, just was helpful for testing).

[wolfjagger]

There are two changes #1289 makes that I do not:

Select.vue:748 they change from this.$refs['deselectButtons'] to document.querySelectorAll('.vs__deselect').
Select.vue:985 they change from this.$options.propsData.hasOwnProperty('reduce'); to typeof this.reduce === 'function';.

I assume both of these changes are for good reason, but I have not looked into it.

[sagalbot]

This was a ton of valuable work @wolfjagger and I really appreciate it! Do you recall why typescript needed to be added as a dev dep?

[j2L4e]

Select.vue:985 they change from this.$options.propsData.hasOwnProperty('reduce'); to typeof this.reduce === 'function';.

this.$options.props.hasOwnProperty('reduce') always throws, because $options.props doesn't have a prototype (at least for Vue 3.1.1). And if it worked, it would always evaluate to true, because the reduce prop is defined on the very component.
typeof this.reduce === 'function' checks if there's a function passed in as a prop. Which seems odd, because it has a default of () => {}.

isTrackingValues () could really use a comment on how it actually determines if it isTrackingValues