Update .NET Core getting started docs to VS2019

[BethMassi]

https://docs.microsoft.com/en-us/dotnet/core/get-started - all docs currently say/show Visual Studio 2017. These need to be updated to Visual Studio 2019

List of high-pri articles to be updated:

• https://docs.microsoft.com/en-us/dotnet/core/get-started
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/with-visual-studio
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/vb-with-visual-studio (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/debugging-with-visual-studio (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/publishing-with-visual-studio (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/library-with-visual-studio (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/vb-library-with-visual-studio (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/testing-library-with-visual-studio (Maira)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/consuming-library-with-visual-studio (Maira)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/using-on-mac-vs-full-solution (Bill)
• https://docs.microsoft.com/en-us/dotnet/core/tutorials/index
• https://docs.microsoft.com/en-us/dotnet/csharp/getting-started/ (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/how-to-set-environment-variables-for-the-visual-studio-command-line (Bill)
• https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/nostdlib-compiler-option (Bill)
• https://docs.microsoft.com/en-us/dotnet/fsharp/get-started/install-fsharp (Genevieve)
• https://docs.microsoft.com/en-us/dotnet/fsharp/get-started/get-started-visual-studio (Genevieve)
• TOC

edit by @mairaw: added article list

[BillWagner]

Assigned to @mairaw and I for now. We can more to someone else, if appropriate.

[mairaw]

Thanks @BethMassi. This has been on my backlog but I didn't have a tracking issue. We also want to make sure it matches the content for the videos /cc @JaySing

[mairaw]

I've added a prioritized list above. I've started a release branch for this because a lot of the Core tutorials are intertwined. Other minor updates I'm making directly to master.

Articles I didn't add to the list above:

• Desktop tutorials in the framework folder (/cc @Thraka)
• Architecture guide pages - there are a few e-book articles that still cover VS 2017. @nishanil would have to comment on a plan for those.