feat: add ES6 module export

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

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

const socket = io("/");
```

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

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

const socket = io("/");

// or
import { Manager } from "socket.io-client";

const manager = new Manager();
const socket = manager.socket("/");
```

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

package.json

@@ -13,7 +13,17 @@
     "dist/",
     "build/"
   ],
+  "type": "commonjs",
   "main": "./build/index.js",
+  "exports": {
+    "./package.json": "./package.json",
+    "./dist/socket.io.js": "./dist/socket.io.js",
+    "./dist/socket.io.js.map": "./dist/socket.io.js.map",
+    ".": {
+      "import": "./wrapper.mjs",
+      "require": "./build/index.js"
+    }
+  },
   "types": "./build/index.d.ts",
   "dependencies": {
     "backo2": "1.0.2",

wrapper.mjs

@@ -0,0 +1,4 @@
+import io from "./build/index.js";
+
+export const Manager = io.Manager;
+export default io;