Add -native support to PowerShell script mill.ps1

[lefou]

See

• update mill.bat to handle -native extension #4290
• Update bootstrap script to allow native Mill launchers #4273

[lihaoyi]

I think it's worth discussing whether we want to support the mill.ps1 launcher at all. I currently don't see any benefit, and "it uses a more modern scripting language" is really more a downside (e.g. causing version-compatibility issues that @ayewo reported). If the .bat launcher does everything we need, and Windows isn't going to remove .bat any time in the foreseeable future, then we should just stick with that