Add support for Solution Filters (.slnf)

[baronfel]

Currently the .NET SDK, DevKit, and other tools that want to interact with solutions correctly have to manually implement for Solution Filter files.

• .NET SDK (via dotnet test):
  • there are other places where we need to add support aside from this
• ionide/proj-info (F# tooling/VSCode)
• DevKit
• MSBuild

And surely more! This is a common tooling scenario that would benefit from a centralized implementation - in addition many of these existing parsers parse the solution in a very inefficient way: they load the entire solution and then apply the mask from the filter to the loaded solution. In an ideal world, the filter could be applied during solution parsing to limit the total amount of work done.