fix(types): $t types

[mitjans]

This PR addresses 2 problems with Typescript types:

• $t does not have TypeScript auto-completions #1215: By removing the type Path from the key: Key | ResourceKeys | Path, we get type inference to work again and suggest valid translations when using globally injected $t. Since the type Key extends string already extends from string, it shouldn't cause any regression. For the type inference to work when using $t, you have to extend the module types:

   import en from '@src/assets/locales/en.json';
   
   type Messages = typeof en;
   
   declare module 'vue-i18n' {
     export interface DefineLocaleMessage extends Messages {}
   }

• When fixing the first issue, I found that nested keys are treated as translations by themselves. For example:

   {
     "this": {
       "is": {
         "a": {
           "nested": {
             "key": "This is a nested key"
           }
         }
       }
     }
   }

  When having this en.json locale file, type inference will suggest a combination of all keys:
  
  But this is not correct, only this.is.a.nested.key should be suggested. I've created two new types to solve this:

1. JsonPaths: Will return leaf keys only, and not suggest in-between keys from a JSON or Javascript object. I've called it Json because this should be used when extracting translations from JSON files.

2. TranslationsPaths: This will simply remove the first level of a JSON or JS object and return the JsonPaths from the second level onwards. This is used for translations coming directly from the composer, where the top-level key is the language.

I've tried not to modify existing types to avoid breaking something, and I just created these 2 new types. Here're two images of the new behavior both in $t and when using the composer:

[kazupon]

Thank you for your contribution!
Wow! Great works!

Your PR will improve i18n DX!

Thanks!