Update the build scripts to support building subfolders or subgroups of projects

* Add build scripts for ci
* Remove obsolete scripts
* Add flags like --test and --pack to control running just test or packaging
* Add flags like --managed and --native to control building sub-types of projects
* Remove KoreBuild bootstrapper flags
* Update to extensions 3.0.0-preview.18619.1 (needed to get a fix for aspnet/Extensions#815 to make this change work on MSBuild.exe)