Update package.json script to work on any OS

[Falke-Design]

Hi,

I work on a windows machine and it is really hard to contribute because the scripts in the package.json are made for Unix-OS and I wasn't able to run them. Additionally I have not understand what they really do, maybe they can be simplified? The best would be a CONTRIBUTER.md file with an explanation.

For #130 i used as workaround npx webpack -c webpack.config.js

[DanielJDufour]

Seems like cross-env could work. It's in maintenance mode, so I'm not 100% sure it's the best choice. Is there an alternative? I'm open to using cross-env if there isn't a better alternative.

https://www.npmjs.com/package/cross-env

Credit to avnir-cs for introducing me to cross-env via https://github.com/GeoTIFF/georaster/pull/68/files