Config variables broken in Windows

Problem/Motivation

When attempting to run the npm build scripts (npm run dev, npm run prod, etc), the following error is encountered:

$ npm run dev

> dev
> cross-env NODE_ENV=development postcss $npm_package_config_inputFile -o $npm_package_config_outputFile

Input Error: You must pass a valid list of files to parse
npm ERR! code 1
...

I can confirm the following: Node Version is 12.16.1, NPM Version is 7.5.2

Here is a stack overflow post that describes this known cross-environment issue: https://stackoverflow.com/questions/42166632/how-to-use-npm-config-varia...

Steps to reproduce

1. Clone a copy of the theme repository to a Windows dev environment
2. Install all dependencies with npm install command
3. Attempt to run npm run dev
4. See following error: Input Error: You must pass a valid list of files to parse

Proposed resolution

There are a few solutions that are proposed in the above Stack Overflow post.
I also found the following:

https://www.npmjs.com/package/cross-var
This would appear to be the lowest friction update, as it would simply replace our existing cross-env usage.

https://www.genui.com/resources/env-variables-json
This would require a little more work. While we would be able to stick with the current dependency of cross-env, we would need to add a .env file, and move the variables there instead. The upside to this is that it would separate the concerns and allow more flexibility for multiple environments.

I don't have a strong opinion either way, but I do think it is important to ensure that we have good cross-platform support in order to allow wide adoption of this powerful starter kit.