Create visual studio projects that are vs2019 compliant.
* nuget_build.ps1 :
Introduce a new argument that will define how we implement the nuget
build. For now we accept 201{9,7.5.3} as possible argument values.

* pugixml_vs2019{,_static}.vcxproj :
Add two visual studio projects that build pugi with the latest SDK and
build tools

* appveyor.yml
- Add Visual Studio 2019 to build targets
- Add Visual Studio 201{9,3,5} to build_scripts. And call
  nuget_build.ps1 with a new argument.
- Add Visual Studio 2019 to the test_scripts.