chore: add types to exports field to be compatible with nodenext module resolution (socketio#4228)

See [1] for detail, in `nodenext` module resolution it requires a
`types` field in `exports` with full filename including extension.

[1]: microsoft/TypeScript#46770 (comment)

Author: otakustay

package.json

@@ -27,7 +27,8 @@
   "main": "./dist/index.js",
   "exports": {
     "import": "./wrapper.mjs",
-    "require": "./dist/index.js"
+    "require": "./dist/index.js",
+    "types": "./dist/index.d.ts"
   },
   "types": "./dist/index.d.ts",
   "license": "MIT",