Provide a metalsmith.env method that abstracts the OS & environment Metalsmith is running in.
An example of what it could look like in a first iteration:

metalsmith.env({       // setter
  platform: 'win32',
  process: 'node',
  version: '16.8.0'
})
metalsmith.env()       // getter
/** returns:
* {
*   platform: 'win32',
*   process: 'node',
*   version: '16.8.0'
* }
**/

This would primarily:

• make it possible to cover all unit test cases on a single OS and single node version.
• provide the base for an additional metalsmith env CLI command that would print out the metalsmith env for debugging purposes and attaching to issues

In a later iteration the option fs: require('fs') could be provided which would open up avenues for using alternative file systems in node-agnostic environments (eg browser LocalStorage or a completely custom system). Though this is a far cry from being implemented and would require other modules like path to be available in those environments too. Most plugins would also be incompatible with this