lint typescript using eslint

[aladdin-add]

refs: https://eslint.org/blog/2019/01/future-typescript-eslint

just bring it up.

[tunnckoCore]

it was possible and good enough even before with just babel-eslint.

[saadq]

I didn't use ESLint for TypeScript before because there were some limitations with rules that had to be fixed with React, but it looks like those are fixed now. Now, I think using ESLint is the way to go considering that TSLint's API is a lot more annoying to work with, it deprecated some important rules like no-unused-variable, and the fact that it will be a lot easier to create editor extensions if I'm only using ESLint.