feat: add ES6 module export

Both CommonJS and ES6 import are now supported:

- with `{ "type": "commonjs" }` in the package.json file

```js
const io = require("socket.io")(8080);
// or
const { Server } = require("socket.io");
const io = new Server(8080);
```

- with `{ "type": "module" }`

```js
import { Server } from "socket.io";

const io = new Server(8080);
```

Related: https://nodejs.org/api/packages.html#packages_dual_commonjs_es_module_packages

package.json

@@ -14,7 +14,12 @@
   "files": [
     "dist/"
   ],
+  "type": "commonjs",
   "main": "./dist/index.js",
+  "exports": {
+    "import": "./wrapper.mjs",
+    "require": "./dist/index.js"
+  },
   "types": "./dist/index.d.ts",
   "license": "MIT",
   "repository": {

wrapper.mjs

@@ -0,0 +1,3 @@
+import io from "./dist/index.js";
+
+export const Server = io.Server;