fix: restore the default export

The default export was accidentally removed in [1].

Note: that being said, users are encouraged to use the named exports,
because the default export has a different meaning for CommonJS and ES
modules users.

```js
// BAD
import io from "socket.io-client";

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

Related:

- https://github.com/socketio/socket.io-client/issues/1505
- https://github.com/socketio/socket.io-client/issues/1507
- https://github.com/socketio/socket.io-client/issues/1508

[1]: https://github.com/socketio/socket.io-client/commit/16b65698aed766e1e645c78847f2e91bfc5b6f56