Add twoslash support

[danvk]

PR Checklist

• Addresses an existing issue: closes Support twoslash syntax (^?) #43
• That issue was marked as status: accepting prs

Overview

Adds support for the following syntax:

const square = (x: number) => x * x;
const four = square(2);
//    ^? const four: number

Multiline types look like:

const vector = {
  x: 3,
  y: 4,
};
vector
// ^? const vector: {
//        x: number;
//        y: number;
//    }

A comment line is a continuation of the expected type if it starts with the exact same sequence of spaces and // as the ^? line (with ^? replaced with more spaces).

This also includes a fixer. I've only enabled it for twoslash-style assertions, but enabling it for $ExpectType would be trivial. This might really improve the dev experience for DefinitelyTyped contributors!

Here are some GIFs of it in action.

Single line fix:

More complex fix that involves multiple files / a multiline type:

[danvk]

I have a line and a column. I want to get the node at that position. Surely there's an easier way than this?!

[danvk]

This should be good to go! I imagine @JoshuaKGoldberg is busy putting the final touches on his book, so no rush on reviewing. But I would love to start using this in my project: https://github.com/danvk/crudely-typed/blob/main/test/type/select.typetest.ts

[JoshuaKGoldberg]

Haha you're right, I am a bit swamped this week (sorry!). But I hope to review as soon as I can!

[JoshuaKGoldberg]

We're scheduled to go over this on Twitch tomorrow 😎 https://www.twitch.tv/joshuakgoldberg/schedule?seriesID=c53bbfdd-92bd-487b-bb14-1ffc5965c191

[JoshuaKGoldberg]

Looks great, thanks so much for sending this! ✨

Nothing too major, mostly just requesting changes we talked about on the stream.

[danvk]

Thanks for the review, Josh! I've addressed all feedback. I think we should be good to go!

[JoshuaKGoldberg]

This is wonderful, thanks @danvk! ✨