Add an autocomplete feature for when typing "t:" to display a list of translation values.

[panoply]

@haroldao

I'm making movements on this one. I don't leverage t: so I need some additional contexts to ensure completions cover all occurrences. Am I missing anything here:

1. Within Section {% schema %} tags
2. Within settings_schema.json
3. Within .json templates
4. Within *-group.json sections

Am I right in my understanding that translation t: prefixes index a single locales .schema.json file? Any further insight etc will be a huge help bud.

Thanks.