1.0 for Twoslash

[orta]

Wraps up my final 1.0 blocker #962 which means I you can make a code sample like:

```ts twoslash
// @filename: Component.tsx
import React from "react"

export function Hello() {
  return (
    <div>
      <h1>Hello World</h1>
    </div>
  )
}

// @filename: index.ts
import { Hello } from "./Component"
console.log(Hello)
```

Without a bunch of faff.

Basically the underlaying vfs will first look for virtual files then it will think that you have a subfolder in your repo's root with this source code. It will then correctly look up through your repo's node_modules for the right types via the normal TS routing.