What problem does this feature solve?

It would be nice with code autocomplete on the Vue SFC Playground.

So it can autocomplete HTML elements, HTML attributes, Vue-specific attributes, and JavaScript.

Example the user type v- and it autocompletes v-if and v-for, etc.

What does the proposed API look like?

N/A

Maybe this PR can solve your problem.

I will close this because it is a problem with vuejs/repl, and you should open an issue in vuejs/repl.