Node 12 "async" annotations are lost when using this package

Node 12 added some limited support for async stack traces. I'm impressed that this package supports them, but it removes a useful annotation from them:

Given this file:

```js
function p() {
return new Promise(resolve => setTimeout(resolve, 0))
}

async function f() {
await p()
throw Error("")
}

async function main() {
await f()
}

main()
.catch(console.error)
```

The stack traces without registering `source-map-support` is:

```
Error
at f (/private/tmp/async-stack-traces/index.js:9:9)
at async main (/private/tmp/async-stack-traces/index.js:13:3)
```

With `source-map-support` registered is:

```
Error:
at f (/private/tmp/async-stack-traces/index.js:9:9)
at main (/private/tmp/async-stack-traces/index.js:13:3)
```

I think the `async` annotation there is useful, as this stack trace is not the actual one as seen by v8, but an augmented one. This augmentation doesn't always work, and it would be surprising to the user if they look like normal stack traces.