Cross-Platform Testing

This release ensures tests are run on ubuntu-latest, macos-latest, windows-latest, using Python versions 3.11, 3.12.0-rc.3. In order for tests to pass on Windows, a minor change needed to be made to Scripts.run_script() so that function will no longer raise a FileNotFoundError if shutil.which() does not find the executable for a given script command. This was necessary because unit tests make liberal use of the echo command, which is a built-in command (not an executable file) on Windows. Moreover, the dev CLI should not enforce this restriction; instead, it now defers to subprocess.run().