Added support for node version checking

[Iamshankhadeep]

fixes #108.

[JonnyBurger]

Thanks! This works!
I would still like to perfect it a little more:

• Let's make the check into a function and call it as the first thing in packages/cli/src/index.tsx. Benefit: We don't need to load ts-node twice which is quite slow. Also the check will run before you start the CLI and not after install, this is a little bit better because the node version can change inbetween.

• By adding to the engines field in package.json, it will hard-fail when your node version is too low. So people will probably not even see the warning we put because NPM is failing. Let's inline the requirement so it's just a warning and not an error (for now)

[JonnyBurger]

Thanks a lot! 💙