feat: Use cross-spawn instead of adding .cmd

[tbarlow12]

What did you implement:

Instead of manually checking for the operating system, this uses the well known package cross-spawn (142M downloads per month) to spawn the process across operating systems.

How did you implement it:

Replaced the call to child_process.spawn with cross-spawn.spawn. Also added jest-os-detection that will only test the spawn calls the way that they should be expected on that native os rather than defining a property on the process object

How can we verify it:

Run sls offline. It will spawn on any operating system. Run npm test on mac, linux or windows. You'll see that 8 tests are skipped on any OS (each OS has 4 tests)

Todos:

Note: Run npm run test:ci to run all validation checks on proposed changes

• [x] Ensure there are no lint errors.
  Validate via npm run lint
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• [x] Write tests and confirm existing functionality is not broken.
  Validate via npm test
• [x] Write documentation
• [x] Provide verification config / commands / resources
• [x] Enable "Allow edits from maintainers" for this PR
• [x] Update the messages below

Is this ready for review?: YES Is it a breaking change?: NO

[pjlittle]

Looks good - much better way of handling OS matrix. 👍