Vue3 compatibility

[lzurbriggen]

• Updated vue-class-component to 8.0.0-beta.4 for Vue3 compatibility
• InjectKey is not exported anymore, had to be defined locally
• PropOptions is indirectly exported via Prop
• @Compontent was renamed to @Options in vue-class-component. I imported it as Component so the vue-property-decorator API doesn't change. Maybe this should be aliased to also allow @Options.

If someone else is working on this, feel free to close this PR.

(This should probably only be merged after [email protected] is stable)

[LifeIsStrange]

Will this allow us to use @watch and @provide and all other decorators like before ?
Can we already use this PR, is it functionally working ?

[LifeIsStrange]

@kaorun343

[lzurbriggen]

I was a little too optimistic, barely anything works at the moment and I probably won't have time to fix it up in the near future.

@LifeIsStrange I think this mainly depends on how extensible vue-class-components's setup function is, I think that could be easy to do if this proposal is implemented.

Closing this for now. I also think this should wait until vue-class-component stabilizes.