build: Add script to build NewLang.pyz

env.ps1: Us python -m pip to avoid weird bugs ...

The Python.Python.3 package on Windows seems to break if you try to
update pip using the pip command. Maybe because the pip binary is in use
during the upgrade?

env: Add domut for mutmut testing

env.ps1: Prefix commands with 'do' to avoid namespacing

env.ps1: Add Windows environment for development