chore: migrate to vue 3

[raimund-schluessler]

This migrates the package to vue 3. It basically pulls in the beta branch from https://github.com/sagalbot/vue-select/tree/beta onto our adjustments. However, I did that manually and fixed some issues along the way.
I did not really care to adjust the docs, but the component itself and the tests work fine.
Also, I don't have yarn, so someone who has it installed needs to create the yarn.lock file. Or we just migrate to npm, which might be better anyway.

I tested the library in the nextcloud/vue docs vue 3 branch, and it works just fine as far as I can tell.

Of course, we wouldn't merge into master yet, but since I don't have the appropriate rights here, I couldn't create any branch here.

[GretaD]

+1 to migrate to npm

[Pytal]

Rights granted @raimund-schluessler :)

[Pytal]

Also might make sense to use something like https://github.com/vueuse/vue-demi for single branch maintenance

[raimund-schluessler]

Also might make sense to use something like https://github.com/vueuse/vue-demi for single branch maintenance

Not sure about this. Vue 2 will be EOL in 3 months, so the better solution is to migrate to Vue 3 and drop support for 2 completely, I think.

[raimund-schluessler]

Rights granted @raimund-schluessler :)

Thanks. How should we proceed here? Should I create a next branch from master and create a PR against it, as we do it in the nextcloud/vue repo?

[Pytal]

Thanks. How should we proceed here? Should I create a next branch from master and create a PR against it, as we do it in the nextcloud/vue repo?

Precisely :)

[raimund-schluessler]

Closed in favor of #25.