fix: improve IVueI18n interface

[kazupon]

No description provided.

[rchl]

I have some more potential improvements:

• make declare class VueI18n extend the interface to avoid having to duplicate the same properties/methods twice
• change readonly $i18n: VueI18n & IVueI18n; in Vue interface augmentation to just readonly $i18n: IVueI18n; since the interface should have everything needed now.

I'm not 100% sure that it's safe to do that. At least for nuxt-i18n it would be a breaking change and I would have to align with it. But I think it's the right thing to do.

[kazupon]

well, vue-i18n typing interface need to more improve.
perhaps, it is made breaking change.
I think that we need to more testing with using tsd.
https://github.com/SamVerschueren/tsd

[rchl]

For nuxt-i18n I just have a test source file that uses types and is compiled by tsc. If it fails then something is wrong with types. Not as fancy and explicit as that testing framework but should be enough.