Convert to ES6, Promises, async and await.

[johnfn]

Converts the entire project to use ES6 Typescript, and converts all promise based code (that I could find) to use async/await.

This makes all of the Promise-based code we had before look a lot nicer. Compare e.g. test/mode/modeInsert.test.ts, which used to be an awful mess of .then(), with how nice it looks now! Since a good chunk of the VSC API is promise-based, I think this will help beautify a lot of our code in the future.

There is a drawback to this approach, which is that until microsoft/vscode#2778 gets resolved, I had to disable some of the ES6 features we were using that are currently hidden behind node flags. These are:

• Default parameters - function foo(x: number = 6)
• Rest arguments - function foo(...x: number[])
• The default export - export default function foo()

I don't see these things as a huge loss compared to the gain of async/await. The frustrating thing is that if you do e.g. use default arguments, you get a fairly obscure error message during runtime that, if you've never seen it before, could be pretty confusing. If the VSC team gets back to us quickly on microsoft/vscode#2778 it won't be a big issue, but if not we could catch it by wrapping import statements in a try catch and giving a more helpful error message.

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I also took the opportunity to make a few minor stylistic changes:

• For modes, we were naming functions in TitleCase, which was inconsistent with the rest of our naming conventions. I changed it to camelCase.
• Use for ... of in some places. for ... of is objectively better than for (let i = 0; i < arr.length; i++) { let obj = arr[i]... and subjectively better than _.forEach because it is prettier. 😄

Hopefully the above isn't contentious and I can sneak it in this PR 😉

[jpoon]

Yay! Thank you. I've been always meaning to do this! Although, looks to be a merge conflict, likely caused I just merged #138

[johnfn]

@jpoon 👍 Just fixed up the merge conflicts, should be good to go!

[jpoon]

@johnfn dammit. I should really merge this PR first. Conflicts again after merging your other one :(

[johnfn]

@jpoon Totally unacceptable.

[johnfn]

...But I fixed it up anyways. 👍