Add TS support and Async/Await-like features

[Vadorequest]

This starter is great and helped me bootstrap my extension quite fast, thanks!

I got slowed down due to the lack of Async/Await support, though.
I tried to use https://github.com/mozilla/webextension-polyfill#basic-setup but I'm getting stuck with the configuration.

Also, I saw it was possible to use TypeScript, and I wish this starter would support it by default, it's so much more comfortable to use than plain JS.

What's your opinion on those 2 features?