chore: automatically replace var with let or const

[DavideViolante]

• add eslint rule no-var
• add eslint rule prefer-const
• run npm run lint --fix to automatically fix the code

Note: I didn't fix any code manually, just ran eslint.

[brianc]

I really like this...I'm a bit worried about breaking other PRs so lemme go through & merge a few, then I'll merge this. Any additional outstanding PRs should be able to aut-fix themselves w/ yarn lint:fix which is 👍