Refactor backend to use Typescript

Seems like types could help understand the code, based off comments like this: https://github.com/rithmschool/numbers_api/blob/master/models/data.js#L1-L19

Plenty of resources out there on Express+TS, here's one example: this could be useful: https://developer.okta.com/blog/2018/11/15/node-express-typescript

(Could also build the React frontend in TS as part of #53)