Update to Vue 3

[terax6669]

I've made this. It's a minimal amount of work required for it to run on Vue 3 + some doc updates. Bear with me on this, this is the first time I have anything to do with a "plugin" or "library" of any (frontend) sort.

Things that still need work:

1. When running npm install there's a bunch of deprecation notices, vodal complains about vue 2.5 dependency, etc.
2. I haven't been able to run the tests for some reason. Maybe because I'm on Windows?

> @riophae/vue-treeselect@0.4.0 lint:js C:\Users\T\Desktop\Dev\www\_pr\vue-treeselect
> eslint --ext .js --ext .vue --cache --cache-location node_modules/.cache/eslint --rule 'no-console: 2' .


Oops! Something went wrong! :(

ESLint: 6.8.0.

No files matching the pattern "2'" were found.

3. At runtime you can see in the console:

runtime-core.esm-bundler.js?5c40:38 [Vue warn]: Failed to resolve component: transition-group
runtime-core.esm-bundler.js?5c40:38 [Vue warn]: Failed to resolve component: transition

I have no idea why. According to https://v3.vuejs.org/guide/transitions-list.html#list-entering-leaving-transitions these tags should be working. This is however just a warning, they are rendered as HTML tags and the component is functional! 🎉

Note for anyone wokring with this:
When I was trying to import from src or from dist I couldn't get this to work when node_modules was present.

runtime-core.esm-bundler.js?5c40:38 [Vue warn]: Missing ref owner context. ref cannot be used on hoisted vnodes. A vnode with ref must be created inside the render function.

At first I thought it was a bug in one of the JSX packages, but by chance I tried removing the node_modules folder during npm run serve and I noticed that those files were in use. When I got rid of the folder all of the ref errors in the console were also gone WTF. When the folder is present all the library's dependencies are loaded from node_modules/@riophae/vue-treeselect/node_modules causing weird errors. WTF. I've literally spent a little over a day trying to figure out those ref errors before stumbling upon the solution by accident. If someone can explain this to me please do. Restore sanity to this world.

[terax6669]

#422

[apuatcfbd]

A package ported from this is working fine with Vue 3 @tkmam1x/vue3-treeselect

[albizeka]

Even though the library is not maintained, its one of the great select libraries in vue, also @tkmam1x/vue3-treeselect seems to work fine on my side