Support single-file .NET apps

[bwateratmsft]

See https://devblogs.microsoft.com/dotnet/announcing-dotnet-run-app/

I tested and already see that dotnet build -getProperty:foo does not work here--so this may not even be possible. But it does work on RC2 so...

[bwateratmsft]

Also, only the SDK images can run these, not the runtime ones.

[danegsta]

Looks like PublishAot is on by default with file based apps, but /p:PublishAot=false worked on Windows to produce an image.