refactor: export typescript declarations for the commonjs build

Related:

- socketio/socket.io#4621 (comment)
- #129

tsconfig.json

@@ -3,7 +3,7 @@
     "outDir": "build/cjs/",
     "target": "es2018", // Node.js 10 (https://github.com/microsoft/TypeScript/wiki/Node-Target-Mapping)
     "module": "commonjs",
-    "declaration": false
+    "declaration": true
   },
   "include": [
     "./lib/**/*"