Fix scripts for yarn start. Update devDependencies

[hidde-vb]

I noticed that yarn start was missing from the script in package.json. I've added a script that reflects the current readme.

Also bumped babel-loader to 8.0.5 because it's needed react-scripts 3.0.0.

Thanks!

[swyxio]

thanks! unfortunately we want to keep the yarn start separate from yarn start:lambda to ensure compatibility for netlify dev :) i'll just update the readme instead

[hidde-vb]

Thanks for taking a look at my PR! The package npm-run-all is not used anymore then?

A simple solution might be to rename the script to yarn start:all or yarn start:stack or something so the script can be kept. And it would only require a small update in the readme.

[swyxio]

yeah its not needed for netlify-dev but you can still use it.