Extend MSBuild binding mechanism to solution properties

In the article https://github.com/dotnet/templating/wiki/Binding-and-project-context-evaluation#binding-to-msbuild-properties it is mentioned that "It is possible to bind symbols to MSBuild properties of the current project."

Upon testing I realized that this really only works for templates running inside a project folder, but will not work for templates running inside the root solution folder.

This would be useful to access properties in `Directory.Build.props` for example for creating documentation pages or other top-level content items which might benefit from accessing global solution properties such as `` or ``.