Add top level await in repl

[esprehn]

The repl doesn't run in an async function which means you can't do ex.

var x = await db.getStuff();

and instead have to use .then() or wrap your line in an async IIFE which is ugly.

See also:
https://twitter.com/umaar/status/897408737694109697

[blakeembrey]

Is it supported in the node.js REPL? Duplicate of #245. Please search before opening an issue.

[blakeembrey]

Feel free to follow up in the linked issue when nodejs/node#13209 gets closed. Ideally we don't have to do anything and it just works, but since there's transpiling that might not be true. I just don't want to deviate too much from what node.js supports.