AppVeyor matrix build

[campersau]

Summary: Builds vs2015, vs2017 and vs2019 on AppVeyor

Changes: #3197 requires matrix builds on appveyor so I have enabled it for the existing supported visual studio versions and can be extended later to support netcore as well.

How Has This Been Tested?
https://ci.appveyor.com/project/campersau/cefsharp/builds/34263683

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Updated documentation

Checklist:

• Tested the code(if applicable)
• Commented my code
• Changed the documentation(if applicable)
• New files have a license disclaimer
• The formatting is consistent with the project (project supports .editorconfig)

[AppVeyorBot]

✅ Build CefSharp 84.3.30-CI3624 completed (commit 16873b82e6 by @campersau)

[amaitland]

The plan is to just have the matrix build vs2015 and the new net core solution.

Building the VS2017 and vs2019 targets is problematic as each creates Nuget packages, what exactly gets pushed to myget?

We're also using AppVeyor as a free service so I'd rather keep the usage of the service to only what's nessicary.

Appreciate the effort though 👍