Add express typings

[darenliang]

Installing @top-gg/sdk inside of a TypeScript project results in the following error when building:

$ npm run build

> test@1.0.0 build /test
> tsc --build tsconfig.json

node_modules/@top-gg/sdk/dist/structs/Webhook.d.ts:1:49 - error TS2307: Cannot find module 'express' or its corresponding type declarations.

1 import { Request, Response, NextFunction } from 'express';
                                                  ~~~~~~~~~


Found 1 error.

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! test@1.0.0 build: `tsc --build tsconfig.json`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the test@1.0.0 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

This PR adds express typings and updates deps.