Port to TypeScript

[danielnixon]

1. ✅ Add a tsc build step (using raw js files)
2. ✅ Add type-coverage build step and corresponding badge
3. ✅ Port js files to ts
4. Enable tsc strict mode
5. Increase type coverage as high as possible
6. Upgrade to TypeScript 4.1
7. Enable noUncheckedIndexedAccess (requires TypeScript 4.1)
8. Enable eslint and https://github.com/danielnixon/eslint-config-typed-fp

[danielnixon]

Groundwork: 237db68