fix: restore the default export (bis)

The previous commit, while successfully restoring support for:

```js
const socket = require("socket.io-client")(...);
```

breaks for some other cases:

- socketio/socket.io#4128
- #1509

According to [1], we should use `export = `, but this is not supported
by module "esnext":

> Export assignment cannot be used when targeting ECMAScript modules

So we'll go for this ugly workaround, at least until we remove the
default export in the next major release.

[1]: https://www.typescriptlang.org/docs/handbook/modules.html#export--and-import--require

lib/index.ts

@@ -80,10 +80,6 @@ Object.assign(lookup, {
   connect: lookup,
 });
 
-if (typeof module !== "undefined") {
-  module.exports = lookup;
-}
-
 /**
  * Protocol version.
  *

postcompile.sh

@@ -6,3 +6,6 @@ cp ./support/package.esm.json ./build/esm/package.json
 cp -r ./build/esm/ ./build/esm-debug/
 
 sed -i '/debug(/d' ./build/esm/*.js
+
+# for backward compatibility with `const socket = require("socket.io-client")(...)`
+echo -e '\nmodule.exports = lookup;' >> ./build/cjs/index.js