Improvement to offline packaging

[gregg-miskelly]

This PR makes various improvements to our offline package support in preparation for switching to that as our primary means of distribution.

More significant changes:

1. Added support for the missing supported platforms
2. Switched to using the same schema as VS Code for the names of platforms
3. Stop dropping the debugger install complete file. The install complete file is used to do some first run OS validation logic, so it should not have been dropped by the offline installer.
4. Lots of error handling improvements:
   • Add a try/catch around doOfflinePackage so we could know which package failed
   • Detect and fail if a package download fails
   • Detect and fail if the vsce process exits with a non-zero exit code
   • Detect and fail if the vsce process fails to create the file we expect it to
5. Fix offline package creation on Linux-x64. This would fail due to multiple files with the same name but different casing.
6. Fix incorrect check in debugger activation code

A few minor changes too:

1. Used more async APIs
2. Move the win32 check up to the start instead of failing each package
3. All the debugger references were to the '.NET Core' debugger and I dropped the 'Core' from this.

[JoeRobich]

CC: @vzarytovskii @nohwnd