Return/intercept output from build export

It would be nice is tsup returned the compiled output when used with the named `build`export. I leverage this fantastic little module within in a build tool and need access to the transformed output. Ideally, being able to intercept the output before it is written:

```js
import { build } from 'tsup';

build({ /* options */ }).then(code => {
console.log(code) // code should return a string or buffer of output
})
```

As of right now the build export returns `void` - Some low level support here would be greatly appreciated. Thanks.