refactor: expose the ESM build with debug (bis)

This reverts the previous commit ([1]), and expose the ESM build that
includes the debug package on a subpath:

```js
import { io } from "socket.io-client/debug";
```

Reference: https://nodejs.org/api/packages.html#subpath-exports

Related: https://github.com/socketio/socket.io-client/issues/1586

[1]: https://github.com/socketio/socket.io-client/commit/781d753a626d01e675056a2ff4e27f5dd599564f