Add support for reading/parsing .slnx file formats.

[IEvangelist]

Add support for reading/parsing .slnx file formats.

Example:

<Solution>
  <Folder Name="/Solution Items/">
    <File Path="Directory.Build.props" />
  </Folder>
  <Project Path="ClassLibrary1\ClassLibrary1.csproj" />
  <Project Path="ConsoleApp1\ConsoleApp1.csproj" />
</Solution>