Initial implemenation of a fluent API for generating Visual Studio solutions

[jeffkl]

Fixes #47

This uses the new https://github.com/microsoft/vs-solutionpersistence library which handles both .sln and .slnx.

This adds support for fluent calls like

ProjectCreator project1 = ProjectCreator.Templates.SdkCsproj(path: Path.Combine("project1", "project1.csproj"));

SolutionCreator.Templates.DotNet(Path.Combine("solution1.sln"))
    .Project(project1)
    .Save();

Solutions are saved automatically for build and all projects are also saved since they need to be on disk.