support i18n in ComponentOptions

[chesnelg]

It is currently not possible to declare i18n messages at component level.
In plain javascript syntax, this is achieved using the i18n.messages field - as described https://kazupon.github.io/vue-i18n/guide/component.html#translation-in-functional-component.

It would be great to be able to pass i18n field to the ComponentOptions interface definition to allow the following:
@Component( { i18n: { messages { ... } } )
The functionality is already in place, but the current type definition doesn't allow it.

[ktsn]

It should be declared on vue-i18n side.
/cc @kazupon

Here is useful information that how to extend vue’s component option type.
https://vuejs.org/v2/guide/typescript.html#Augmenting-Types-for-Use-with-Plugins