Updates the guide for writing `ng add` schematics to include the new authoring utilities. I've also removed the `NodePackageInstallTask` because it's redundant.
PR Close#50667
Use the cross-platform `copyfiles` package instead of using `cp` which fails on Windows machines when copying files in the `postbuild` script of the "schematics-for-libraries" example.
PR Close#30966
Kristiyan Kostadinov