refactor(project): improve typescript next/branch

[suddenlyGiovanni]

As for PR: https://github.com/kentcdodds/react-hooks/pull/112#issue-554331129

Story

As a trainee, I want the reference files ./src/final-ts/** to be written in TypeScript React so that I can better understand React's API.”

AC:

1. Project should be configured to support both TypeScript and Javascript
2. The trainee must be able to work in the ./src/exercises folder with no consequences.
3. All source code in ./src/final-ts/should be refactored to TypeScript.
4. Only syntax changes are allowed.
   4.1. The codebase should remain as close as possible to @kentcdodds's initial implementation.
   4.2. Should avoid distracting the user with complicated type signatures
5. The code changes should expose only the best practices for production-ready React/Typescript codebase.
6. All tests must pass!

refactor(project): to ts: initial commit
refactor(exercise-01): to ts
refactor(exercise-02): to ts
refactor(exercise-03): to ts
refactor(exercise-04): to ts
refactor(exercise-04): to ts: extra credit 01: prop getters
refactor(exercise-05): to ts
refactor(exercise-05): to ts: extra credit 01: default state reducer
refactor(exercise-05): to ts: extra credit 02: state reducer action types
refactor(exercise-06): to ts
refactor(exercise-06): to ts: extra credit 01: add read only warning
refactor(exercise-06): to ts: extra credit 02: add a controlled state warning
refactor(exercise-06): to ts: extra credit 03: extract warnings to a custom hook
refactor(exercise-06): to ts: extra credit 04: don’t warn in production

[Aprillion]

I was just thinking about how this exercise should look in TypeScript and there are 2 ways that made the most sense to me:

• a) if we want ToggleOn, ToggleOff, and ToggleButton to be "exported" with the optional props, then this line needs to be something like React.cloneElement(child, {on, toggle, ...child.props}) to make the type signatures truthful
• b) if we don't want to accept on and toggle props (similar to the original version), then we need different types for the "exported" component than the actual implementation

[kentcdodds]

Actually, when I get around to this exercise, I think I'm going to do something like this: https://twitter.com/markdalgleish/status/1370951759284162565

[kentcdodds]

Thanks again for this PR. I've finished my own changes to the next branch now :)