add property decorators

[kaorun343]

Hi.

As a lot of the developers use TS and Vue together, I think vue-class-component and vue-property-decorator should be unified.

So, I would like to add some decorators in my repository to this repository.

[sinisterstumble]

vue-property-decorator depends on reflect-metadata which offloads the cost of developer convenience to the client. I would argue that there are many that are happy with just vue-class-component. It would be nice to retain the choice the developers have with the two modules, especially that vue-class-component defines a way to extend its functionality.

[ktsn]

I think just making reflect-metadata optional dependency would fine in that case.

[kaorun343]

OK.

[metapragma]

If this gets merged, and the decorators from vue-property-decorator aren't used, will they be treeshaken away?

[sinisterstumble]

@paerallax good point. Something like this would be nice

import { Compontent, Watch } from 'vue-class-component' @Component class MyComp extends Vue { @Watch('child') onChildChanged(val: string, oldVal: string) { } }

In this case index.ts should not import Watch on line 6

[kaorun343]

I will add more commits since February due to my research at university, thanks.

[kaorun343]

Done.

Review this PR, please.

[kaorun343]

@ktsn
Could you review this PR?