[babel] use babel-runtime+transform-runtime to support async functions

[spalger]

Using async functions and targeting runtimes that don't support generators means that eui must depend on regenerator. Without it the test/syncify.js helper throws ReferenceError: regeneratorRuntime is not defined when installed as a node modules.

This pr includes the transform-runtime babel plugin which will rewrite access to the regeneratorRuntime global with calls to require("babel-runtime/regenerator"), which is now listed as a dependency of eui. This does add a fairly sizable dependency for one async function, but since Kibana is also using regenerator it should only get included in the bundles once.

Alternatively we can disallow async functions in eslint config and convert the syncify helper to use standard promises.

[pugnascotia]

Why does EUI need async functions? I'm not against them, but if it's just to avoid a few promises, then it feels like overkill.

[chrisronline]

We can just remove syncify from EUI if it’s the only use case and we don’t expect more.

[bevacqua]

-1, would strive to keep EUI as lean as we can. It's only getting larger :)

[kimjoar]

PR for removing syncify: #306. syncify is no longer needed because of updates in Jest.

I'll close this. Let's just re-open if we want to reconsider async/await at some point.