refactor: expose the ESM build with debug (bis)

This reverts the previous commit ([1]), and expose the ESM build that
includes the debug package on a subpath:

```js
import { Socket } from "engine.io-client/debug";
```

Reference: https://nodejs.org/api/packages.html#subpath-exports

Related: https://github.com/socketio/socket.io-client/issues/1586

[1]: fe4d93a

package.json

@@ -13,10 +13,13 @@
     ".": {
       "import": {
         "node": "./build/esm-debug/index.js",
-        "development": "./build/esm-debug/index.js",
         "default": "./build/esm/index.js"
       },
       "require": "./build/cjs/index.js"
+    },
+    "./debug": {
+      "import": "./build/esm-debug/index.js",
+      "require": "./build/cjs/index.js"
     }
   },
   "types": "build/esm/index.d.ts",