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:

- #1505
- #1507
- #1508

[1]: 16b6569

Author: darrachequesne

lib/index.ts

@@ -80,6 +80,10 @@ Object.assign(lookup, {
   connect: lookup,
 });
 
+if (typeof module !== "undefined") {
+  module.exports = lookup;
+}
+
 /**
  * Protocol version.
  *
@@ -101,4 +105,5 @@ export {
   SocketOptions,
   lookup as io,
   lookup as connect,
+  lookup as default,
 };