Convert project to typescript

[AllanWang]

There will be a bunch of tricks to shorten the code, make it less error prone, and make it easier to navigate

https://hackernoon.com/writing-better-reduxs-code-with-typescript-b7a3c1209bca

[AllanWang]

Open ended unions; type switching based on key

microsoft/TypeScript#26277

Actually works so long as you declare all possible types

https://www.patrickdunn.org/advanced-types-in-typescript/

[AllanWang]

Mapped types, readonly, list comprehension like mutation

https://www.typescriptlang.org/docs/handbook/advanced-types.html