Add alias to be able to process .nuspec files

### What You Are Seeing?
Currently in cake there is no way to work directly with .nuspec files to prepare them for nuget pack, or choco pack.

### What is Expected?
The only way a nuspec can be created is when running NuGetPack or ChocolateyPack. Would be great to be possible to create/update .nuspec without running those commands. This will allow for example to create a .nuspec file and pass it as parameter for dotnet pack

It's related to this https://github.com/cake-build/cake/issues/1037

### What version of Cake are you using?
0.30.0

### Are you running on a 32 or 64 bit system?
64-bit

### What environment are you running on? Windows? Linux? Mac?
Windows, Linux

Start by locating the NuGetPack and ChocolateyPack commands and review the related issue #1037 to understand the existing nuspec-generation behavior. Check how their inputs are exposed and identify the entry point for processing a standalone .nuspec file. Done means a .nuspec can be created or updated independently and passed to dotnet pack without invoking those commands.

Written by the indexing model from the issue text.