fix(typings): ensure compatibility with TypeScript 3.x (#4259)

Labeled tuple elements were added in TypeScript 4.0.

Reference: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#labeled-tuple-elements

Related: https://github.com/socketio/socket.io/commit/44e20ba5bfcb54f5bbe678fc1bf7f30bb9d90274