Make package.json scripts work on Windows

[c-w]

Currently there are several bashisms in the various package.json scripts which makes it impossible to work on boardgame.io natively on Windows without using a Linux emulation layer such as WSL. This commit replaces the bashisms with cross-platform Node commands.

Checklist

• Use a separate branch in your local repo (not master).
• Test coverage is 100% (or you have a story for why it's ok).

[nicolodavis]

Thanks for taking the time to do this!! Will take a closer look in a few days (busy this week).

[nicolodavis]

@c-w Made a few changes. Let me know if they look good to you.

The main addition is to run "build" in prestart so you can just do:

npm install
npm start

when you first clone the repo.

[c-w]

@nicolodavis Changes LGTM.

[delucis]

Thanks Clemens!