Ignore locally-built `*.nupkg` files

Building NuGet packages locally with `NuGet.exe pack <name>.nuspec` generates a `<name>.<version>.nupkg` file which should never be versioned.

VisualStudio.gitignore

@@ -129,8 +129,9 @@ publish/
 ## passwords
 *.pubxml
 
-# NuGet Packages Directory
+# NuGet Packages
 packages/*
+*.nupkg
 ## TODO: If the tool you use requires repositories.config
 ## uncomment the next line
 #!packages/repositories.config