Migrate codebase to Typescript

**Is your feature request related to a problem? Please describe.**

We currently use javascript + [type definitions](https://github.com/anuraghazra/github-readme-stats/blob/master/src/fetchers/types.d.ts). I, however, think migrating the project to typescript will significantly improve the development experience since it allows us to catch errors earlier.

**Extra information**

There were some attempts in the past that people can build upon if they want to take on this issue:

- https://github.com/anuraghazra/github-readme-stats/pull/1288
- https://github.com/anuraghazra/github-readme-stats/pull/832

Additionally, @HwangTaehyun translated part of the codebase to typescript when creating https://github.com/HwangTaehyun/github-contributor-stats.