Add some nice warnings if plugins happen to return promises instead of sync values.

[loganfsmyth]

This does nothing for current users. The intention with this is just a bit of future-proofing. I think at some point we'll want to add support for plugins that are async, either as a top-level promise-returning function like an async function(){}, or with async handler functions of like pre, post, parserOverride, or generatorOverride.

This PR does 2 things:

• Add clear error messages to each of those call sites, so if they do return a promise, the user will have at least some hint of what to do. That way if we allow this eventually in 7.x and someone writes a plugin using it, and then a user runs it on an older 7.x release, they'll get some kind of feedback.
• This adds an api.async() function that would allow a plugin to check if async logic is allowed in plugins. Where in the future it might return true if used via a Babel transform that allows async logic, like transform("", () => {}).

I'm imagining that in the future we might for instance have users doing

import asyncPlugin from "./async";
import syncPlugin from "./sync";
export default function(api, ...args) {
  return api.async() ? asyncPlugin(api, ...args) : syncPlugin(api, ...args);
}

which is certainly just a guess, but at least if we do go that route, plugins could rely on it being there in early 7.x versions too.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/5800/

[hzoo]

cool, I like the progress of setting this up for later