Simplify development of vsix

[sayedihashimi]

Some ideas after talking with Mads today.

1. Make convention to drop .nupkg in ProjectTemplates folder in source folder
2. Make nuget pkg that users can add to get the target added
3. On build generate the .pkgdef and add to pipeline
   For the path use RootNamespace-AssemblyName
4. When installing the package, tranform vsixmanifest to add the asset tag